Published On: February 24ᵗʰ, 2020 00:29

Routing Configuration Guide, Cisco IOS XE Everest 16.6.x (Catalyst 3850 Switches)

Information About Configuring IP Unicast Routing

This module describes how to configure IP Version 4 (IPv4) unicast routing on the switch.

A switch stack operates and appears as a single router to the rest of the routers in the network. Basic routing functions like static routing are available with both the IP Base feature set and the IP Services feature set. To use advanced routing features and other routing protocols, you must have the IP Services feature set enabled on the standalone switch or on the active switch.


Note

In addition to IPv4 traffic, you can also enable IP Version 6 (IPv6) unicast routing and configure interfaces to forward IPv6 trafficif the switch or switch stack is running the IP Base or IP Services feature set .


Information About IP Routing

In some network environments, VLANs are associated with individual networks or subnetworks. In an IP network, each subnetwork is mapped to an individual VLAN. Configuring VLANs helps control the size of the broadcast domain and keeps local traffic local. However, network devices in different VLANs cannot communicate with one another without a Layer 3 device (router) to route traffic between the VLAN, referred to as inter-VLAN routing. You configure one or more routers to route traffic to the appropriate destination VLAN.

Figure 1. Routing Topology Example. This figure shows a basic routing topology. Switch A is in VLAN 10, and Switch B is in VLAN 20. The router has an interface in each VLAN.

When Host A in VLAN 10 needs to communicate with Host B in VLAN 10, it sends a packet addressed to that host. Switch A forwards the packet directly to Host B, without sending it to the router.

When Host A sends a packet to Host C in VLAN 20, Switch A forwards the packet to the router, which receives the traffic on the VLAN 10 interface. The router checks the routing table, finds the correct outgoing interface, and forwards the packet on the VLAN 20 interface to Switch B. Switch B receives the packet and forwards it to Host C.

Types of Routing

Routers and Layer 3 switches can route packets in these ways:

  • By using default routing

  • By using preprogrammed static routes for the traffic

  • By dynamically calculating routes by using a routing protocol

Default routing refers to sending traffic with a destination unknown to the router to a default outlet or destination.

Static unicast routing forwards packets from predetermined ports through a single path into and out of a network. Static routing is secure and uses little bandwidth, but does not automatically respond to changes in the network, such as link failures, and therefore, might result in unreachable destinations. As networks grow, static routing becomes a labor-intensive liability.

Switches running the LAN base feature set support 16 user-configured static routes, in addition to any default routes used for the management interface. The LAN base image supports static routing only on SVIs.

Dynamic routing protocols are used by routers to dynamically calculate the best route for forwarding traffic. There are two types of dynamic routing protocols:

  • Routers using distance-vector protocols maintain routing tables with distance values of networked resources, and periodically pass these tables to their neighbors. Distance-vector protocols use one or a series of metrics for calculating the best routes. These protocols are easy to configure and use.

  • Routers using link-state protocols maintain a complex database of network topology, based on the exchange of link-state advertisements (LSAs) between routers. LSAs are triggered by an event in the network, which speeds up the convergence time or time required to respond to these changes. Link-state protocols respond quickly to topology changes, but require greater bandwidth and more resources than distance-vector protocols.

Distance-vector protocols supported by the switch are Routing Information Protocol (RIP), which uses a single distance metric (cost) to determine the best path and Border Gateway Protocol (BGP), which adds a path vector mechanism. The switch also supports the Open Shortest Path First (OSPF) link-state protocol and Enhanced IGRP (EIGRP), which adds some link-state routing features to traditional Interior Gateway Routing Protocol (IGRP) to improve efficiency.


Note

On a switch or switch stack, the supported protocols are determined by the software running on the active switch. If the active switch is running the IP base feature set, only default routing, static routing and RIP are supported. If the switch is running the LAN base feature set, you can configure 16 static routes on SVIs. All other routing protocols require the IP services feature set.


IP Routing and Switch Stacks

A switch stack appears to the network as a single switch, regardless of which switch in the stack is connected to a routing peer.

The active switch performs these functions:

  • It initializes and configures the routing protocols.

  • It sends routing protocol messages and updates to other routers.

  • It processes routing protocol messages and updates received from peer routers.

  • It generates, maintains, and distributes the distributed Cisco Express Forwarding (dCEF) database to all stack members. The routes are programmed on all switches in the stack bases on this database.

  • The MAC address of the active switch is used as the router MAC address for the whole stack, and all outside devices use this address to send IP packets to the stack.

  • All IP packets that require software forwarding or processing go through the CPU of the active switch.

Stack members perform these functions:

  • They act as routing standby switches, ready to take over in case they are elected as the new active switch if the active switch fails.

  • They program the routes into hardware.

If a active switch fails, the stack detects that the active switch is down and elects one of the stack members to be the new active switch. During this period, except for a momentary interruption, the hardware continues to forward packets with no active protocols.

However, even though the switch stack maintains the hardware identification after a failure, the routing protocols on the router neighbors might flap during the brief interruption before the active switch restarts. Routing protocols such as OSPF and EIGRP need to recognize neighbor transitions. The router uses two levels of nonstop forwarding (NSF) to detect a switchover, to continue forwarding network traffic, and to recover route information from peer devices:

  • NSF-aware routers tolerate neighboring router failures. After the neighbor router restarts, an NSF-aware router supplies information about its state and route adjacencies on request.

  • NSF-capable routers support NSF. When they detect a active switch change, they rebuild routing information from NSF-aware or NSF-capable neighbors and do not wait for a restart.

The switch stack supports NSF-capable routing for OSPF and EIGRP.

Upon election, the new active switch performs these functions:

  • It starts generating, receiving, and processing routing updates.

  • It builds routing tables, generates the CEF database, and distributes it to stack members.

  • It uses its MAC address as the router MAC address. To notify its network peers of the new MAC address, it periodically (every few seconds for 5 minutes) sends a gratuitous ARP reply with the new router MAC address.


    Note

    If you configure the persistent MAC address feature on the stack and the active switch changes, the stack MAC address does not change for the configured time period. If the previous active switch rejoins the stack as a member switch during that time period, the stack MAC address remains the MAC address of the previous active switch.


  • It attempts to determine the reachability of every proxy ARP entry by sending an ARP request to the proxy ARP IP address and receiving an ARP reply. For each reachable proxy ARP IP address, it generates a gratuitous ARP reply with the new router MAC address. This process is repeated for 5 minutes after a new active switch election.


    Note

    When a active switch is running the IP services feature set, the stack can run all supported protocols, including Open Shortest Path First (OSPF), and Enhanced IGRP (EIGRP) . If the active switch fails and the new elected active switch is running the IP base or LAN base feature set, these protocols will no longer run in the stack.



    Caution

    Partitioning of the switch stack into two or more stacks might lead to undesirable behavior in the network.


If the switch is reloaded, then all the ports on that switch go down and there is a loss of traffic for the interfaces involved in routing, despite NSF/SSO capability.

Classless Routing

By default, classless routing behavior is enabled on the Device when it is configured to route. With classless routing, if a router receives packets for a subnet of a network with no default route, the router forwards the packet to the best supernet route. A supernet consists of contiguous blocks of Class C address spaces used to simulate a single, larger address space and is designed to relieve the pressure on the rapidly depleting Class B address space.

In the figure, classless routing is enabled. When the host sends a packet to 120.20.4.1, instead of discarding the packet, the router forwards it to the best supernet route. If you disable classless routing and a router receives packets destined for a subnet of a network with no network default route, the router discards the packet.

Figure 2. IP Classless Routing

In the figure , the router in network 128.20.0.0 is connected to subnets 128.20.1.0, 128.20.2.0, and 128.20.3.0. If the host sends a packet to 120.20.4.1, because there is no network default route, the router discards the packet.

Figure 3. No IP Classless Routing

To prevent the Device from forwarding packets destined for unrecognized subnets to the best supernet route possible, you can disable classless routing behavior.

Address Resolution

You can control interface-specific handling of IP by using address resolution. A device using IP can have both a local address or MAC address, which uniquely defines the device on its local segment or LAN, and a network address, which identifies the network to which the device belongs.


Note

In a switch stack, network communication uses a single MAC address and the IP address of the stack.


The local address or MAC address is known as a data link address because it is contained in the data link layer (Layer 2) section of the packet header and is read by data link (Layer 2) devices. To communicate with a device on Ethernet, the software must learn the MAC address of the device. The process of learning the MAC address from an IP address is called address resolution . The process of learning the IP address from the MAC address is called reverse address resolution .

The Device can use these forms of address resolution:

  • Address Resolution Protocol (ARP) is used to associate IP address with MAC addresses. Taking an IP address as input, ARP learns the associated MAC address and then stores the IP address/MAC address association in an ARP cache for rapid retrieval. Then the IP datagram is encapsulated in a link-layer frame and sent over the network. Encapsulation of IP datagrams and ARP requests or replies on IEEE 802 networks other than Ethernet is specified by the Subnetwork Access Protocol (SNAP).

  • Proxy ARP helps hosts with no routing tables learn the MAC addresses of hosts on other networks or subnets. If the Device (router) receives an ARP request for a host that is not on the same interface as the ARP request sender, and if the router has all of its routes to the host through other interfaces, it generates a proxy ARP packet giving its own local data link address. The host that sent the ARP request then sends its packets to the router, which forwards them to the intended host.

The Device also uses the Reverse Address Resolution Protocol (RARP), which functions the same as ARP does, except that the RARP packets request an IP address instead of a local MAC address. Using RARP requires a RARP server on the same network segment as the router interface. Use the ip rarp-server address interface configuration command to identify the server.

For more information on RARP, see the Cisco IOS Configuration Fundamentals Configuration Guide

Proxy ARP

Proxy ARP, the most common method for learning about other routes, enables an Ethernet host with no routing information to communicate with hosts on other networks or subnets. The host assumes that all hosts are on the same local Ethernet and that they can use ARP to learn their MAC addresses. If a Device receives an ARP request for a host that is not on the same network as the sender, the Device evaluates whether it has the best route to that host. If it does, it sends an ARP reply packet with its own Ethernet MAC address, and the host that sent the request sends the packet to the Device, which forwards it to the intended host. Proxy ARP treats all networks as if they are local and performs ARP requests for every IP address.

ICMP Router Discovery Protocol

Router discovery allows the Device to dynamically learn about routes to other networks using ICMP router discovery protocol (IRDP). IRDP allows hosts to locate routers. When operating as a client, the Device generates router discovery packets. When operating as a host, the Device receives router discovery packets. The Device can also listen to Routing Information Protocol (RIP) routing updates and use this information to infer locations of routers. The Device does not actually store the routing tables sent by routing devices; it merely keeps track of which systems are sending the data. The advantage of using IRDP is that it allows each router to specify both a priority and the time after which a device is assumed to be down if no further packets are received.

Each device discovered becomes a candidate for the default router, and a new highest-priority router is selected when a higher priority router is discovered, when the current default router is declared down, or when a TCP connection is about to time out because of excessive retransmissions.

UDP Broadcast Packets and Protocols

User Datagram Protocol (UDP) is an IP host-to-host layer protocol, as is TCP. UDP provides a low-overhead, connectionless session between two end systems and does not provide for acknowledgment of received datagrams. Network hosts occasionally use UDP broadcasts to find address, configuration, and name information. If such a host is on a network segment that does not include a server, UDP broadcasts are normally not forwarded. You can remedy this situation by configuring an interface on a router to forward certain classes of broadcasts to a helper address. You can use more than one helper address per interface.

You can specify a UDP destination port to control which UDP services are forwarded. You can specify multiple UDP protocols. You can also specify the Network Disk (ND) protocol, which is used by older diskless Sun workstations and the network security protocol SDNS.

By default, both UDP and ND forwarding are enabled if a helper address has been defined for an interface. The description for the ip forward-protocol interface configuration command in the Cisco IOS IP Command Reference, Volume 1 of 3: Addressing and Services lists the ports that are forwarded by default if you do not specify any UDP ports.

Broadcast Packet Handling

After configuring an IP interface address, you can enable routing and configure one or more routing protocols, or you can configure the way the Device responds to network broadcasts. A broadcast is a data packet destined for all hosts on a physical network. The Device supports two kinds of broadcasting:

  • A directed broadcast packet is sent to a specific network or series of networks. A directed broadcast address includes the network or subnet fields.

  • A flooded broadcast packet is sent to every network.


    Note

    You can also limit broadcast, unicast, and multicast traffic on Layer 2 interfaces by using the storm-control interface configuration command to set traffic suppression levels.


Routers provide some protection from broadcast storms by limiting their extent to the local cable. Bridges (including intelligent bridges), because they are Layer 2 devices, forward broadcasts to all network segments, thus propagating broadcast storms. The best solution to the broadcast storm problem is to use a single broadcast address scheme on a network. In most modern IP implementations, you can set the address to be used as the broadcast address. Many implementations, including the one in the Device, support several addressing schemes for forwarding broadcast messages.

IP Broadcast Flooding

You can allow IP broadcasts to be flooded throughout your internetwork in a controlled fashion by using the database created by the bridging STP. Using this feature also prevents loops. To support this capability, bridging must be configured on each interface that is to participate in the flooding. If bridging is not configured on an interface, it still can receive broadcasts. However, the interface never forwards broadcasts it receives, and the router never uses that interface to send broadcasts received on a different interface.

Packets that are forwarded to a single network address using the IP helper-address mechanism can be flooded. Only one copy of the packet is sent on each network segment.

To be considered for flooding, packets must meet these criteria. (Note that these are the same conditions used to consider packet forwarding using IP helper addresses.)

  • The packet must be a MAC-level broadcast.

  • The packet must be an IP-level broadcast.

  • The packet must be a TFTP, DNS, Time, NetBIOS, ND, or BOOTP packet, or a UDP specified by the ip forward-protocol udp global configuration command.

  • The time-to-live (TTL) value of the packet must be at least two.

A flooded UDP datagram is given the destination address specified with the ip broadcast-address interface configuration command on the output interface. The destination address can be set to any address. Thus, the destination address might change as the datagram propagates through the network. The source address is never changed. The TTL value is decremented.

When a flooded UDP datagram is sent out an interface (and the destination address possibly changed), the datagram is handed to the normal IP output routines and is, therefore, subject to access lists, if they are present on the output interface.

In the Device, the majority of packets are forwarded in hardware; most packets do not go through the Device CPU. For those packets that do go to the CPU, you can speed up spanning tree-based UDP flooding by a factor of about four to five times by using turbo-flooding. This feature is supported over Ethernet interfaces configured for ARP encapsulation.

How to Configure IP Routing

By default, IP routing is disabled on the Device, and you must enable it before routing can take place.

In the following procedures, the specified interface must be one of these Layer 3 interfaces:

  • A routed port: a physical port configured as a Layer 3 port by using the no switchport interface configuration command.

  • A switch virtual interface (SVI): a VLAN interface created by using the interface vlan vlan_id global configuration command and by default a Layer 3 interface.


    Note

    On enabling ip routing, the VLAN configured as SVI will also learn broadcast ARP requests which are not self destined.


  • An EtherChannel port channel in Layer 3 mode: a port-channel logical interface created by using the interface port-channel port-channel-number global configuration command and binding the Ethernet interface into the channel group.


    Note

    The switch does not support tunnel interfaces for unicast routed traffic.


All Layer 3 interfaces on which routing will occur must have IP addresses assigned to them.


Note

A Layer 3 switch can have an IP address assigned to each routed port and SVI.

The number of routed ports and SVIs that you can configure is limited to , exceeding the recommended number and volume of features being implemented might impact CPU utilization because of hardware limitations.


Configuring routing consists of several main procedures:

  • To support VLAN interfaces, create and configure VLANs on the Device or switch stack, and assign VLAN membership to Layer 2 interfaces. For more information, see the "Configuring VLANs” chapter.

  • Configure Layer 3 interfaces.

  • Enable IP routing on the switch.

  • Assign IP addresses to the Layer 3 interfaces.

  • Enable selected routing protocols on the switch.

  • Configure routing protocol parameters (optional).

How to Configure IP Addressing

A required task for configuring IP routing is to assign IP addresses to Layer 3 network interfaces to enable the interfaces and allow communication with the hosts on those interfaces that use IP. The following sections describe how to configure various IP addressing features. Assigning IP addresses to the interface is required; the other procedures are optional.

  • Default Addressing Configuration

  • Assigning IP Addresses to Network Interfaces

  • Configuring Address Resolution Methods

  • Routing Assistance When IP Routing is Disabled

  • Configuring Broadcast Packet Handling

  • Monitoring and Maintaining IP Addressing

Default IP Addressing Configuration

Table 1. Default Addressing Configuration

Feature

Default Setting

IP address

None defined.

ARP

No permanent entries in the Address Resolution Protocol (ARP) cache.

Encapsulation: Standard Ethernet-style ARP.

Timeout: 14400 seconds (4 hours).

IP broadcast address

255.255.255.255 (all ones).

IP classless routing

Enabled.

IP default gateway

Disabled.

IP directed broadcast

Disabled (all IP directed broadcasts are dropped).

IP domain

Domain list: No domain names defined.

Domain lookup: Enabled.

Domain name: Enabled.

IP forward-protocol

If a helper address is defined or User Datagram Protocol (UDP) flooding is configured, UDP forwarding is enabled on default ports.

Any-local-broadcast: Disabled.

Spanning Tree Protocol (STP): Disabled.

Turbo-flood: Disabled.

IP helper address

Disabled.

IP host

Disabled.

IRDP

Disabled.

Defaults when enabled:

  • Broadcast IRDP advertisements.

  • Maximum interval between advertisements: 600 seconds.

  • Minimum interval between advertisements: 0.75 times max interval

  • Preference: 0.

IP proxy ARP

Enabled.

IP routing

Disabled.

IP subnet-zero

Disabled.

Assigning IP Addresses to Network Interfaces

An IP address identifies a location to which IP packets can be sent. Some IP addresses are reserved for special uses and cannot be used for host, subnet, or network addresses. RFC 1166, “Internet Numbers,” contains the official description of IP addresses.

An interface can have one primary IP address. A mask identifies the bits that denote the network number in an IP address. When you use the mask to subnet a network, the mask is referred to as a subnet mask. To receive an assigned network number, contact your Internet service provider.

Procedure

  Command or Action Purpose
Step 1

enable

Example:


Device> enable


Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:


Device# configure terminal


Enters global configuration mode.

Step 3

interface interface-id

Example:


Device(config)# interface gigabitethernet 1/0/1

Enters interface configuration mode, and specifies the Layer 3 interface to configure.

Step 4

no switchport

Example:


Device(config-if)# no switchport

Removes the interface from Layer 2 configuration mode (if it is a physical interface).

Step 5

ip address ip-address subnet-mask

Example:


Device(config-if)# ip address 10.1.5.1 255.255.255.0

Configures the IP address and IP subnet mask.

Step 6

no shutdown

Example:


Device(config-if)# no shutdown

Enables the physical interface.

Step 7

end

Example:


Device(config)# end


Returns to privileged EXEC mode.

Step 8

show ip route

Example:


Device# show ip route

Verifies your entries.

Step 9

show ip interface [interface-id]

Example:


Device# show ip interface gigabitethernet 1/0/1

Verifies your entries.

Step 10

show running-config

Example:


Device# show running-config 


Verifies your entries.

Step 11

copy running-config startup-config

Example:


Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Using Subnet Zero

Subnetting with a subnet address of zero is strongly discouraged because of the problems that can arise if a network and a subnet have the same addresses. For example, if network 131.108.0.0 is subnetted as 255.255.255.0, subnet zero would be written as 131.108.0.0, which is the same as the network address.

You can use the all ones subnet (131.108.255.0) and even though it is discouraged, you can enable the use of subnet zero if you need the entire subnet space for your IP address.

Use the no ip subnet-zero global configuration command to restore the default and disable the use of subnet zero.

Procedure
  Command or Action Purpose
Step 1

enable

Example:

Device> enable


Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal


Enters global configuration mode.

Step 3

ip subnet-zero

Example:

Device(config)# ip subnet-zero

Enables the use of subnet zero for interface addresses and routing updates.

Step 4

end

Example:

Device(config)# end


Returns to privileged EXEC mode.

Step 5

show running-config

Example:

Device# show running-config 


Verifies your entries.

Step 6

copy running-config startup-config

Example:

Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Disabling Classless Routing

To prevent the Device from forwarding packets destined for unrecognized subnets to the best supernet route possible, you can disable classless routing behavior.

Procedure
  Command or Action Purpose
Step 1

enable

Example:

Device> enable


Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal


Enters global configuration mode.

Step 3

no ip classless

Example:

Device(config)#no ip classless

Disables classless routing behavior.

Step 4

end

Example:

Device(config)# end


Returns to privileged EXEC mode.

Step 5

show running-config

Example:

Device# show running-config 


Verifies your entries.

Step 6

copy running-config startup-config

Example:

Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Configuring Address Resolution Methods

You can perform the following tasks to configure address resolution.

Defining a Static ARP Cache

ARP and other address resolution protocols provide dynamic mapping between IP addresses and MAC addresses. Because most hosts support dynamic address resolution, you usually do not need to specify static ARP cache entries. If you must define a static ARP cache entry, you can do so globally, which installs a permanent entry in the ARP cache that the Device uses to translate IP addresses into MAC addresses. Optionally, you can also specify that the Device respond to ARP requests as if it were the owner of the specified IP address. If you do not want the ARP entry to be permanent, you can specify a timeout period for the ARP entry.

Procedure
  Command or Action Purpose
Step 1

enable

Example:

Device> enable


Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal


Enters global configuration mode.

Step 3

arp ip-address hardware-address type

Example:

Device(config)# ip 10.1.5.1 c2f3.220a.12f4 arpa

Associates an IP address with a MAC (hardware) address in the ARP cache, and specifies encapsulation type as one of these:

  • arpa —ARP encapsulation for Ethernet interfaces

  • snap —Subnetwork Address Protocol encapsulation for Token Ring and FDDI interfaces

  • sap —HP’s ARP type

Step 4

arp ip-address hardware-address type [alias]

Example:

Device(config)# ip 10.1.5.3 d7f3.220d.12f5 arpa alias

(Optional) Specifies that the switch respond to ARP requests as if it were the owner of the specified IP address.

Step 5

interface interface-id

Example:

Device(config)# interface gigabitethernet 1/0/1

Enters interface configuration mode, and specifies the interface to configure.

Step 6

arp timeout seconds

Example:

Device(config-if)# arp 20000

(Optional) Sets the length of time an ARP cache entry will stay in the cache. The default is 14400 seconds (4 hours). The range is 0 to 2147483 seconds.

Step 7

end

Example:

Device(config)# end


Returns to privileged EXEC mode.

Step 8

show interfaces [interface-id]

Example:

Device# show interfaces gigabitethernet 1/0/1

Verifies the type of ARP and the timeout value used on all interfaces or a specific interface.

Step 9

show arp

Example:

Device# show arp

Views the contents of the ARP cache.

Step 10

show ip arp

Example:

Device# show ip arp

Views the contents of the ARP cache.

Step 11

copy running-config startup-config

Example:

Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Setting ARP Encapsulation

By default, Ethernet ARP encapsulation (represented by the arpa keyword) is enabled on an IP interface. You can change the encapsulation methods to SNAP if required by your network.

To disable an encapsulation type, use the no arp arpa or no arp snap interface configuration command.

Procedure
  Command or Action Purpose
Step 1

enable

Example:

Device> enable


Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal


Enters global configuration mode.

Step 3

interface interface-id

Example:

Device(config)# interface gigabitethernet 1/0/2

Enters interface configuration mode, and specifies the Layer 3 interface to configure.

Step 4

arp {arpa | snap}

Example:

Device(config-if)# arp arpa

Specifies the ARP encapsulation method:

  • arpa —Address Resolution Protocol

  • snap —Subnetwork Address Protocol

Step 5

end

Example:

Device(config)# end


Returns to privileged EXEC mode.

Step 6

show interfaces [interface-id]

Example:

Device# show interfaces

Verifies ARP encapsulation configuration on all interfaces or the specified interface.

Step 7

copy running-config startup-config

Example:

Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Enabling Proxy ARP

By default, the Device uses proxy ARP to help hosts learn MAC addresses of hosts on other networks or subnets.

Procedure
  Command or Action Purpose
Step 1

enable

Example:

Device> enable


Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal


Enters global configuration mode.

Step 3

interface interface-id

Example:

Device(config)# interface gigabitethernet 1/0/2

Enters interface configuration mode, and specifies the Layer 3 interface to configure.

Step 4

ip proxy-arp

Example:

Device(config-if)# ip proxy-arp

Enables proxy ARP on the interface.

Step 5

end

Example:

Device(config)# end


Returns to privileged EXEC mode.

Step 6

show ip interface [interface-id]

Example:

Device# show ip interface gigabitethernet 1/0/2

Verifies the configuration on the interface or all interfaces.

Step 7

copy running-config startup-config

Example:

Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Routing Assistance When IP Routing is Disabled

These mechanisms allow the Device to learn about routes to other networks when it does not have IP routing enabled:

  • Proxy ARP

  • Default Gateway

  • ICMP Router Discovery Protocol (IRDP)

Proxy ARP

Proxy ARP is enabled by default. To enable it after it has been disabled, see the “Enabling Proxy ARP” section. Proxy ARP works as long as other routers support it.

Default Gateway

Another method for locating routes is to define a default router or default gateway. All non-local packets are sent to this router, which either routes them appropriately or sends an IP Control Message Protocol (ICMP) redirect message back, defining which local router the host should use. The Device caches the redirect messages and forwards each packet as efficiently as possible. A limitation of this method is that there is no means of detecting when the default router has gone down or is unavailable.

Procedure
  Command or Action Purpose
Step 1

enable

Example:

Device> enable


Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal


Enters global configuration mode.

Step 3

ip default-gateway ip-address

Example:

Device(config)# ip default gateway 10.1.5.1

Sets up a default gateway (router).

Step 4

end

Example:

Device(config)# end


Returns to privileged EXEC mode.

Step 5

show ip redirects

Example:

Device# show ip redirects

Displays the address of the default gateway router to verify the setting.

Step 6

copy running-config startup-config

Example:

Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

ICMP Router Discovery Protocol (IRDP)

The only required task for IRDP routing on an interface is to enable IRDP processing on that interface. When enabled, the default parameters apply.

You can optionally change any of these parameters. If you change the maxadvertinterval value, the holdtime and minadvertinterval values also change, so it is important to first change the maxadvertinterval value, before manually changing either the holdtime or minadvertinterval values.

Procedure
  Command or Action Purpose
Step 1

enable

Example:

Device> enable


Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal


Enters global configuration mode.

Step 3

interface interface-id

Example:

Device(config)# interface gigabitethernet 1/0/1

Enters interface configuration mode, and specifies the Layer 3 interface to configure.

Step 4

ip irdp

Example:

Device(config-if)# ip irdp

Enables IRDP processing on the interface.

Step 5

ip irdp multicast

Example:

Device(config-if)# ip irdp multicast

(Optional) Sends IRDP advertisements to the multicast address (224.0.0.1) instead of IP broadcasts.

Note 

This command allows for compatibility with Sun Microsystems Solaris, which requires IRDP packets to be sent out as multicasts. Many implementations cannot receive these multicasts; ensure end-host ability before using this command.

Step 6

ip irdp holdtime seconds

Example:

Device(config-if)# ip irdp holdtime 1000

(Optional) Sets the IRDP period for which advertisements are valid. The default is three times the maxadvertinterval value. It must be greater than maxadvertinterval and cannot be greater than 9000 seconds. If you change the maxadvertinterval value, this value also changes.

Step 7

ip irdp maxadvertinterval seconds

Example:

Device(config-if)# ip irdp maxadvertinterval 650

(Optional) Sets the IRDP maximum interval between advertisements. The default is 600 seconds.

Step 8

ip irdp minadvertinterval seconds

Example:

Device(config-if)# ip irdp minadvertinterval 500

(Optional) Sets the IRDP minimum interval between advertisements. The default is 0.75 times the maxadvertinterval . If you change the maxadvertinterval , this value changes to the new default (0.75 of maxadvertinterval ).

Step 9

ip irdp preference number

Example:

Device(config-if)# ip irdp preference 2

(Optional) Sets a device IRDP preference level. The allowed range is –231 to 231. The default is 0. A higher value increases the router preference level.

Step 10

ip irdp address address [number]

Example:

Device(config-if)# ip irdp address 10.1.10.10

(Optional) Specifies an IRDP address and preference to proxy-advertise.

Step 11

end

Example:

Device(config)# end


Returns to privileged EXEC mode.

Step 12

show ip irdp

Example:

Device# show ip irdp

Verifies settings by displaying IRDP values.

Step 13

copy running-config startup-config

Example:

Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Configuring Broadcast Packet Handling

Perform the tasks in these sections to enable these schemes:

  • Enabling Directed Broadcast-to-Physical Broadcast Translation

  • Forwarding UDP Broadcast Packets and Protocols

  • Establishing an IP Broadcast Address

  • Flooding IP Broadcasts

Enabling Directed Broadcast-to-Physical Broadcast Translation

By default, IP directed broadcasts are dropped; they are not forwarded. Dropping IP-directed broadcasts makes routers less susceptible to denial-of-service attacks.

You can enable forwarding of IP-directed broadcasts on an interface where the broadcast becomes a physical (MAC-layer) broadcast. Only those protocols configured by using the ip forward-protocol global configuration command are forwarded.

You can specify an access list to control which broadcasts are forwarded. When an access list is specified, only those IP packets permitted by the access list are eligible to be translated from directed broadcasts to physical broadcasts. For more information on access lists, see the “Information about Network Security with ACLs" section in the Security Configuration Guide.

Procedure
  Command or Action Purpose
Step 1

enable

Example:

Device> enable


Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal


Enters global configuration mode.

Step 3

interface interface-id

Example:

Device(config)# interface gigabitethernet 1/0/2

Enters interface configuration mode, and specifies the interface to configure.

Step 4

ip directed-broadcast [access-list-number]

Example:

Device(config-if)# ip directed-broadcast 103

Enables directed broadcast-to-physical broadcast translation on the interface. You can include an access list to control which broadcasts are forwarded. When an access list, only IP packets permitted by the access list can be translated.

Step 5

exit

Example:

Device(config-if)# exit

Returns to global configuration mode.

Step 6

ip forward-protocol {udp [port] | nd | sdns}

Example:

Device(config)# ip forward-protocol nd

Specifies which protocols and ports the router forwards when forwarding broadcast packets.

  • udp —Forward UPD datagrams.

    port: (Optional) Destination port that controls which UDP services are forwarded.

  • nd —Forward ND datagrams.

  • sdns —Forward SDNS datagrams

Step 7

end

Example:

Device(config)# end


Returns to privileged EXEC mode.

Step 8

show ip interface [interface-id]

Example:

Device# show ip interface

Verifies the configuration on the interface or all interfaces

Step 9

show running-config

Example:

Device# show running-config 


Verifies your entries.

Step 10

copy running-config startup-config

Example:

Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Forwarding UDP Broadcast Packets and Protocols

If you do not specify any UDP ports when you configure the forwarding of UDP broadcasts, you are configuring the router to act as a BOOTP forwarding agent. BOOTP packets carry DHCP information.

Procedure
  Command or Action Purpose
Step 1

enable

Example:

Device> enable


Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal


Enters global configuration mode.

Step 3

interface interface-id

Example:

Device(config)# interface gigabitethernet 1/0/1

Enters interface configuration mode, and specifies the Layer 3 interface to configure.

Step 4

ip helper-address address

Example:

Device(config-if)# ip helper address 10.1.10.1

Enables forwarding and specifies the destination address for forwarding UDP broadcast packets, including BOOTP.

Step 5

exit

Example:

Device(config-if)# exit

Returns to global configuration mode.

Step 6

ip forward-protocol {udp [port] | nd | sdns}

Example:

Device(config)# ip forward-protocol sdns

Specifies which protocols the router forwards when forwarding broadcast packets.

Step 7

end

Example:

Device(config)# end


Returns to privileged EXEC mode.

Step 8

show ip interface [interface-id]

Example:

Device# show ip interface gigabitethernet 1/0/1

Verifies the configuration on the interface or all interfaces.

Step 9

show running-config

Example:

Device# show running-config 


Verifies your entries.

Step 10

copy running-config startup-config

Example:

Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Establishing an IP Broadcast Address

The most popular IP broadcast address (and the default) is an address consisting of all ones (255.255.255.255). However, the Device can be configured to generate any form of IP broadcast address.

Procedure
  Command or Action Purpose
Step 1

enable

Example:

Device> enable


Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal


Enters global configuration mode.

Step 3

interface interface-id

Example:

Device(config)# interface gigabitethernet 1/0/1

Enters interface configuration mode, and specifies the interface to configure.

Step 4

ip broadcast-address ip-address

Example:

Device(config-if)# ip broadcast-address 128.1.255.255

Enters a broadcast address different from the default, for example 128.1.255.255.

Step 5

end

Example:

Device(config)# end


Returns to privileged EXEC mode.

Step 6

show ip interface [interface-id]

Example:

Device# show ip interface

Verifies the broadcast address on the interface or all interfaces.

Step 7

copy running-config startup-config

Example:

Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Flooding IP Broadcasts

Procedure
  Command or Action Purpose
Step 1

enable

Example:

Device> enable


Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal


Enters global configuration mode.

Step 3

ip forward-protocol spanning-tree

Example:

Device(config)# ip forward-protocol spanning-tree

Uses the bridging spanning-tree database to flood UDP datagrams.

Step 4

end

Example:

Device(config)# end


Returns to privileged EXEC mode.

Step 5

show running-config

Example:

Device# show running-config 


Verifies your entries.

Step 6

copy running-config startup-config

Example:

Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Step 7

configure terminal

Example:

Device# configure terminal


Enters global configuration mode.

Step 8

ip forward-protocol turbo-flood

Example:

Device(config)# ip forward-protocol turbo-flood

Uses the spanning-tree database to speed up flooding of UDP datagrams.

Step 9

end

Example:

Device(config)# end


Returns to privileged EXEC mode.

Step 10

show running-config

Example:

Device# show running-config 


Verifies your entries.

Step 11

copy running-config startup-config

Example:

Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Monitoring and Maintaining IP Addressing

When the contents of a particular cache, table, or database have become or are suspected to be invalid, you can remove all its contents by using the clear privileged EXEC commands. The Table lists the commands for clearing contents.

Table 2. Commands to Clear Caches, Tables, and Databases

clear arp-cache

Clears the IP ARP cache and the fast-switching cache.

clear host {name | *}

Removes one or all entries from the hostname and the address cache.

clear ip route {network [mask] | *}

Removes one or more routes from the IP routing table.

You can display specific statistics, such as the contents of IP routing tables, caches, and databases; the reachability of nodes; and the routing path that packets are taking through the network. The Table lists the privileged EXEC commands for displaying IP statistics.

Table 3. Commands to Display Caches, Tables, and Databases

show arp

Displays the entries in the ARP table.

show hosts

Displays the default domain name, style of lookup service, name server hosts, and the cached list of hostnames and addresses.

show ip aliases

Displays IP addresses mapped to TCP ports (aliases).

show ip arp

Displays the IP ARP cache.

show ip interface [interface-id]

Displays the IP status of interfaces.

show ip irdp

Displays IRDP values.

show ip masks address

Displays the masks used for network addresses and the number of subnets using each mask.

show ip redirects

Displays the address of a default gateway.

show ip route [address [mask]] | [protocol]

Displays the current state of the routing table.

show ip route summary

Displays the current state of the routing table in summary form.

How to Configure IP Unicast Routing

Enabling IP Unicast Routing

By default, the Device is in Layer 2 switching mode and IP routing is disabled. To use the Layer 3 capabilities of the Device, you must enable IP routing.

Procedure

  Command or Action Purpose
Step 1

enable

Example:


Device> enable


Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:


Device# configure terminal


Enters global configuration mode.

Step 3

ip routing

Example:


Device(config)# ip routing

Enables IP routing.

Step 4

end

Example:


Device(config)# end


Returns to privileged EXEC mode.

Step 5

show running-config

Example:


Device# show running-config 


Verifies your entries.

Step 6

copy running-config startup-config

Example:


Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Example of Enabling IP Routing

This example shows how to enable IP routingusing RIP as the routing protocol :


Device# configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Device(config)# ip routing
Device(config)# router rip
Device(config-router)# network 10.0.0.0
Device(config-router)# end

What to Do Next

You can now set up parameters for the selected routing protocols as described in these sections:

  • RIP

  • OSPF,

  • EIGRP

  • BGP

  • Unicast Reverse Path Forwarding

  • Protocol-Independent Features (optional)

Information About RIP

The Routing Information Protocol (RIP) is an interior gateway protocol (IGP) created for use in small, homogeneous networks. It is a distance-vector routing protocol that uses broadcast User Datagram Protocol (UDP) data packets to exchange routing information. The protocol is documented in RFC 1058. You can find detailed information about RIP in IP Routing Fundamentals, published by Cisco Press.


Note

RIP is supported in the IP Base Network Essentials feature set.


Using RIP, the Device sends routing information updates (advertisements) every 30 seconds. If a router does not receive an update from another router for 180 seconds or more, it marks the routes served by that router as unusable. If there is still no update after 240 seconds, the router removes all routing table entries for the non-updating router.

RIP uses hop counts to rate the value of different routes. The hop count is the number of routers that can be traversed in a route. A directly connected network has a hop count of zero; a network with a hop count of 16 is unreachable. This small range (0 to 15) makes RIP unsuitable for large networks.

If the router has a default network path, RIP advertises a route that links the router to the pseudonetwork 0.0.0.0. The 0.0.0.0 network does not exist; it is treated by RIP as a network to implement the default routing feature. The Device advertises the default network if a default was learned by RIP or if the router has a gateway of last resort and RIP is configured with a default metric. RIP sends updates to the interfaces in specified networks. If an interface’s network is not specified, it is not advertised in any RIP update.

Summary Addresses and Split Horizon

Routers connected to broadcast-type IP networks and using distance-vector routing protocols normally use the split-horizon mechanism to reduce the possibility of routing loops. Split horizon blocks information about routes from being advertised by a router on any interface from which that information originated. This feature usually optimizes communication among multiple routers, especially when links are broken.

How to Configure RIP

Default RIP Configuration

Table 4. Default RIP Configuration

Feature

Default Setting

Auto summary

Enabled.

Default-information originate

Disabled.

Default metric

Built-in; automatic metric translations.

IP RIP authentication key-chain

No authentication.

Authentication mode: clear text.

IP RIP triggered

Disabled

IP split horizon

Varies with media.

Neighbor

None defined.

Network

None specified.

Offset list

Disabled.

Output delay

0 milliseconds.

Timers basic

  • Update: 30 seconds.

  • Invalid: 180 seconds.

  • Hold-down: 180 seconds.

  • Flush: 240 seconds.

Validate-update-source

Enabled.

Version

Receives RIP Version 1 and 2 packets; sends Version 1 packets.

Configuring Basic RIP Parameters

To configure RIP, you enable RIP routing for a network and optionally configure other parameters. On the Device, RIP configuration commands are ignored until you configure the network number.

Procedure

  Command or Action Purpose
Step 1

enable

Example:


Device> enable


Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:


Device# configure terminal


Enters global configuration mode.

Step 3

ip routing

Example:


Device(config)# ip routing

Enables IP routing. (Required only if IP routing is disabled.)

Step 4

router rip

Example:


Device(config)# router rip

Enables a RIP routing process, and enter router configuration mode.

Step 5

network network number

Example:


Device(config)# network 12

Associates a network with a RIP routing process. You can specify multiple network commands. RIP routing updates are sent and received through interfaces only on these networks.

Note 

You must configure a network number for the RIP commands to take effect.

Step 6

neighbor ip-address

Example:


Device(config)# neighbor 10.2.5.1

(Optional) Defines a neighboring router with which to exchange routing information. This step allows routing updates from RIP (normally a broadcast protocol) to reach nonbroadcast networks.

Step 7

offset-list [access-list number | name] {in | out} offset [type number]

Example:


Device(config)# offset-list 103 in 10

(Optional) Applies an offset list to routing metrics to increase incoming and outgoing metrics to routes learned through RIP. You can limit the offset list with an access list or an interface.

Step 8

timers basic update invalid holddown flush

Example:


Device(config)# timers basic 45 360 400 300

(Optional) Adjusts routing protocol timers. Valid ranges for all timers are 0 to 4294967295 seconds.

  • update —The time between sending routing updates. The default is 30 seconds.

  • invalid —The timer after which a route is declared invalid. The default is 180 seconds.

  • holddown —The time before a route is removed from the routing table. The default is 180 seconds.

  • flush —The amount of time for which routing updates are postponed. The default is 240 seconds.

Step 9

version {1 | 2}

Example:


Device(config)# version 2

(Optional) Configures the switch to receive and send only RIP Version 1 or RIP Version 2 packets. By default, the switch receives Version 1 and 2 but sends only Version 1. 
You can also use the interface commands ip rip {send | receive} version 1 | 2 | 1 2} to control what versions are used for sending and receiving on interfaces.

Step 10

no auto summary

Example:


Device(config)# no auto summary

(Optional) Disables automatic summarization. By default, the switch summarizes subprefixes when crossing classful network boundaries. Disable summarization (RIP Version 2 only) to advertise subnet and host routing information to classful network boundaries.

Step 11

no validate-update-source

Example:


Device(config)# no validdate-update-source

(Optional) Disables validation of the source IP address of incoming RIP routing updates. By default, the switch validates the source IP address of incoming RIP routing updates and discards the update if the source address is not valid. Under normal circumstances, disabling this feature is not recommended. However, if you have a router that is off-network and you want to receive its updates, you can use this command.

Step 12

output-delay delay

Example:


Device(config)# output-delay 8

(Optional) Adds interpacket delay for RIP updates sent.
By default, packets in a multiple-packet RIP update have no delay added between packets. If you are sending packets to a lower-speed device, you can add an interpacket delay in the range of 8 to 50 milliseconds.

Step 13

end

Example:


Device(config)# end


Returns to privileged EXEC mode.

Step 14

show ip protocols

Example:


Device# show ip protocols

Verifies your entries.

Step 15

copy running-config startup-config

Example:


Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Configuring RIP Authentication

RIP Version 1 does not support authentication. If you are sending and receiving RIP Version 2 packets, you can enable RIP authentication on an interface. The key chain specifies the set of keys that can be used on the interface. If a key chain is not configured, no authentication is performed, not even the default.

The Device supports two modes of authentication on interfaces for which RIP authentication is enabled: plain text and MD5. The default is plain text.

Procedure

  Command or Action Purpose
Step 1

enable

Example:


Device> enable


Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:


Device# configure terminal


Enters global configuration mode.

Step 3

interface interface-id

Example:


Device(config)# interface gigabitethernet 1/0/1

Enters interface configuration mode, and specifies the interface to configure.

Step 4

ip rip authentication key-chain name-of-chain

Example:


Device(config-if)# ip rip authentication key-chain trees

Enables RIP authentication.

Step 5

ip rip authentication mode {text | md5}

Example:


Device(config-if)# ip rip authentication mode md5

Configures the interface to use plain text authentication (the default) or MD5 digest authentication.

Step 6

end

Example:


Device(config)# end


Returns to privileged EXEC mode.

Step 7

show running-config

Example:


Device# show running-config 


Verifies your entries.

Step 8

copy running-config startup-config

Example:


Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Configuring Summary Addresses and Split Horizon


Note

In general, disabling split horizon is not recommended unless you are certain that your application requires it to properly advertise routes.


If you want to configure an interface running RIP to advertise a summarized local IP address pool on a network access server for dial-up clients, use the ip summary-address rip interface configuration command.


Note

If split horizon is enabled, neither autosummary nor interface IP summary addresses are advertised.


Procedure

  Command or Action Purpose
Step 1

enable

Example:


Device> enable


Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:


Device# configure terminal


Enters global configuration mode.

Step 3

interface interface-id

Example:


Device(config)# interface gigabitethernet 1/0/1

Enters interface configuration mode, and specifies the Layer 3 interface to configure.

Step 4

ip address ip-address subnet-mask

Example:


Device(config-if)# ip address 10.1.1.10 255.255.255.0

Configures the IP address and IP subnet.

Step 5

ip summary-address rip ip address ip-network mask

Example:


Device(config-if)# ip summary-address rip ip address 10.1.1.30 255.255.255.0

Configures the IP address to be summarized and the IP network mask.

Step 6

no ip split horizon

Example:


Device(config-if)# no ip split horizon

Disables split horizon on the interface.

Step 7

end

Example:


Device(config)# end


Returns to privileged EXEC mode.

Step 8

show ip interface interface-id

Example:


Device# show ip interface gigabitethernet 1/0/1

Verifies your entries.

Step 9

copy running-config startup-config

Example:


Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Configuring Split Horizon

Routers connected to broadcast-type IP networks and using distance-vector routing protocols normally use the split-horizon mechanism to reduce the possibility of routing loops. Split horizon blocks information about routes from being advertised by a router on any interface from which that information originated. This feature can optimize communication among multiple routers, especially when links are broken.


Note

In general, we do not recommend disabling split horizon unless you are certain that your application requires it to properly advertise routes.


Procedure

  Command or Action Purpose
Step 1

enable

Example:


Device> enable


Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:


Device# configure terminal


Enters global configuration mode.

Step 3

interface interface-id

Example:


Device(config)# interface gigabitethernet 1/0/1

Enters interface configuration mode, and specifies the interface to configure.

Step 4

ip address ip-address subnet-mask

Example:


Device(config-if)# ip address 10.1.1.10 255.255.255.0

Configures the IP address and IP subnet.

Step 5

no ip split-horizon

Example:


Device(config-if)# no ip split-horizon

Disables split horizon on the interface.

Step 6

end

Example:


Device(config)# end


Returns to privileged EXEC mode.

Step 7

show ip interface interface-id

Example:


Device# show ip interface gigabitethernet 1/0/1

Verifies your entries.

Step 8

copy running-config startup-config

Example:


Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Configuration Example for Summary Addresses and Split Horizon

In this example, the major net is 10.0.0.0. The summary address 10.2.0.0 overrides the autosummary address of 10.0.0.0 so that 10.2.0.0 is advertised out interface Gigabit Ethernet port 2, and 10.0.0.0 is not advertised. In the example, if the interface is still in Layer 2 mode (the default), you must enter a no switchport interface configuration command before entering the ip address interface configuration command.


Note

If split horizon is enabled, neither autosummary nor interface summary addresses (those configured with the ip summary-address rip router configuration command) are advertised.


Device(config)# router rip
Device(config-router)# interface gigabitethernet1/0/2
Device(config-if)# ip address 10.1.5.1 255.255.255.0
Device(config-if)# ip summary-address rip 10.2.0.0 255.255.0.0
Device(config-if)# no ip split-horizon
Device(config-if)# exit
Device(config)# router rip
Device(config-router)# network 10.0.0.0
Device(config-router)# neighbor 2.2.2.2 peer-group mygroup
Device(config-router)# end


Information About OSPF

OSPF is an Interior Gateway Protocol (IGP) designed expressly for IP networks, supporting IP subnetting and tagging of externally derived routing information. OSPF also allows packet authentication and uses IP multicast when sending and receiving packets. The Cisco implementation supports RFC 1253, OSPF management information base (MIB).


Note

OSPF is supported in IP Base.


The Cisco implementation conforms to the OSPF Version 2 specifications with these key features:

  • Definition of stub areas is supported.

  • Routes learned through any IP routing protocol can be redistributed into another IP routing protocol. At the intradomain level, this means that OSPF can import routes learned through EIGRP and RIP. OSPF routes can also be exported into RIP.

  • Plain text and MD5 authentication among neighboring routers within an area is supported.

  • Configurable routing interface parameters include interface output cost, retransmission interval, interface transmit delay, router priority, router dead and hello intervals, and authentication key.

  • Virtual links are supported.

  • Not-so-stubby-areas (NSSAs) per RFC 1587are supported.

OSPF typically requires coordination among many internal routers, area border routers (ABRs) connected to multiple areas, and autonomous system boundary routers (ASBRs). The minimum configuration would use all default parameter values, no authentication, and interfaces assigned to areas. If you customize your environment, you must ensure coordinated configuration of all routers.


Note

It is not recommended to use OSPF aggressive timers. An OSPF hello timer of less than five seconds is considered aggressive. OSPF and other routing protocols are handled at normal priority and sub second scheduling under high CPU usage conditions in not guaranteed.

BFD control packets are handled with high priority by a separate queue and bfd packets are processed in a high priority thread. BFD is preferred over OSPF for faster convergence.


OSPF Nonstop Forwarding

The Device or switch stack supports two levels of nonstop forwarding (NSF):

OSPF NSF Awareness

When the neighboring router is NSF-capable, the Layer 3 Device continues to forward packets from the neighboring router during the interval between the primary Route Processor (RP) in a router crashing and the backup RP taking over, or while the primary RP is manually reloaded for a non-disruptive software upgrade.

This feature cannot be disabled.

OSPF NSF Capability

supports the OSPFv2 NSF IETF format in addition to the OSPFv2 NSF Cisco format that is supported in earlier releases. For information about this feature, see : NSF—OSPF (RFC 3623 OSPF Graceful Restart).

The also supports OSPF NSF-capable routing for IPv4 for better convergence and lower traffic loss following a stack master change. When a stack master change occurs in an OSPF NSF-capable stack, the new stack master must do two things to resynchronize its link-state database with its OSPF neighbors:

  • Release the available OSPF neighbors on the network without resetting the neighbor relationship.

  • Reacquire the contents of the link-state database for the network.

After a stack master change, the new master sends an OSPF NSF signal to neighboring NSF-aware devices. A device recognizes this signal to mean that it should not reset the neighbor relationship with the stack. As the NSF-capable stack master receives signals from other routes on the network, it begins to rebuild its neighbor list.

When the neighbor relationships are reestablished, the NSF-capable stack master resynchronizes its database with its NSF-aware neighbors, and routing information is exchanged between the OSPF neighbors. The new stack master uses this routing information to remove stale routes, to update the routing information database (RIB), and to update the forwarding information base (FIB) with the new information. The OSPF protocols then fully converge.


Note

OSPF NSF requires that all neighbor networking devices be NSF-aware. If an NSF-capable router discovers non-NSF aware neighbors on a network segment, it disables NSF capabilities for that segment. Other network segments where all devices are NSF-aware or NSF-capable continue to provide NSF capabilities.


Use the nsf OSPF routing configuration command to enable OSPF NSF routing. Use the show ip ospf privileged EXEC command to verify that it is enabled.

For more information, see Cisco Nonstop Forwarding:
http://www.cisco.com/en/US/docs/ios/ha/configuration/guide/ha-nonstp_fwdg.html

OSPF Area Parameters

You can optionally configure several OSPF area parameters. These parameters include authentication for password-based protection against unauthorized access to an area, stub areas, and not-so-stubby-areas (NSSAs). Stub areas are areas into which information on external routes is not sent. Instead, the area border router (ABR) generates a default external route into the stub area for destinations outside the autonomous system (AS). An NSSA does not flood all LSAs from the core into the area, but can import AS external routes within the area by redistribution.

Route summarization is the consolidation of advertised addresses into a single summary route to be advertised by other areas. If network numbers are contiguous, you can use the area range router configuration command to configure the ABR to advertise a summary route that covers all networks in the range.

Other OSPF Parameters

You can optionally configure other OSPF parameters in router configuration mode.

  • Route summarization: When redistributing routes from other protocols. Each route is advertised individually in an external LSA. To help decrease the size of the OSPF link state database, you can use the summary-address router configuration command to advertise a single router for all the redistributed routes included in a specified network address and mask.

  • Virtual links: In OSPF, all areas must be connected to a backbone area. You can establish a virtual link in case of a backbone-continuity break by configuring two Area Border Routers as endpoints of a virtual link. Configuration information includes the identity of the other virtual endpoint (the other ABR) and the nonbackbone link that the two routers have in common (the transit area). Virtual links cannot be configured through a stub area.

  • Default route: When you specifically configure redistribution of routes into an OSPF routing domain, the route automatically becomes an autonomous system boundary router (ASBR). You can force the ASBR to generate a default route into the OSPF routing domain.

  • Domain Name Server (DNS) names for use in all OSPF show privileged EXEC command displays makes it easier to identify a router than displaying it by router ID or neighbor ID.

  • Default Metrics: OSPF calculates the OSPF metric for an interface according to the bandwidth of the interface. The metric is calculated as ref-bw divided by bandwidth, where ref is 10 by default, and bandwidth (bw ) is specified by the bandwidth interface configuration command. For multiple links with high bandwidth, you can specify a larger number to differentiate the cost on those links.

  • Administrative distance is a rating of the trustworthiness of a routing information source, an integer between 0 and 255, with a higher value meaning a lower trust rating. An administrative distance of 255 means the routing information source cannot be trusted at all and should be ignored. OSPF uses three different administrative distances: routes within an area (interarea), routes to another area (interarea), and routes from another routing domain learned through redistribution (external). You can change any of the distance values.

  • Passive interfaces: Because interfaces between two devices on an Ethernet represent only one network segment, to prevent OSPF from sending hello packets for the sending interface, you must configure the sending device to be a passive interface. Both devices can identify each other through the hello packet for the receiving interface.

  • Route calculation timers: You can configure the delay time between when OSPF receives a topology change and when it starts the shortest path first (SPF) calculation and the hold time between two SPF calculations.

  • Log neighbor changes: You can configure the router to send a syslog message when an OSPF neighbor state changes, providing a high-level view of changes in the router.

LSA Group Pacing

The OSPF LSA group pacing feature allows the router to group OSPF LSAs and pace the refreshing, check-summing, and aging functions for more efficient router use. This feature is enabled by default with a 4-minute default pacing interval, and you will not usually need to modify this parameter. The optimum group pacing interval is inversely proportional to the number of LSAs the router is refreshing, check-summing, and aging. For example, if you have approximately 10,000 LSAs in the database, decreasing the pacing interval would benefit you. If you have a very small database (40 to 100 LSAs), increasing the pacing interval to 10 to 20 minutes might benefit you slightly.

Loopback Interfaces

OSPF uses the highest IP address configured on the interfaces as its router ID. If this interface is down or removed, the OSPF process must recalculate a new router ID and resend all its routing information out its interfaces. If a loopback interface is configured with an IP address, OSPF uses this IP address as its router ID, even if other interfaces have higher IP addresses. Because loopback interfaces never fail, this provides greater stability. OSPF automatically prefers a loopback interface over other interfaces, and it chooses the highest IP address among all loopback interfaces.

How to Configure OSPF

Default OSPF Configuration

Table 5. Default OSPF Configuration

Feature

Default Setting

Interface parameters

Cost: 1.

Retransmit interval: 5 seconds.

Transmit delay: 1 second.

Priority: 1.

Hello interval: 10 seconds.

Dead interval: 4 times the hello interval.

No authentication.

No password specified.

MD5 authentication disabled.

Area

Authentication type: 0 (no authentication).

Default cost: 1.

Range: Disabled.

Stub: No stub area defined.

NSSA: No NSSA area defined.

Auto cost

100 Mb/s.

Default-information originate

Disabled. When enabled, the default metric setting is 10, and the external route type default is Type 2.

Default metric

Built-in, automatic metric translation, as appropriate for each routing protocol.

Distance OSPF

dist1 (all routes within an area): 110.
dist2 (all routes from one area to another): 110.
and dist3 (routes from other routing domains): 110.

OSPF database filter

Disabled. All outgoing link-state advertisements (LSAs) are flooded to the interface.

IP OSPF name lookup

Disabled.

Log adjacency changes

Enabled.

Neighbor

None specified.

Neighbor database filter

Disabled. All outgoing LSAs are flooded to the neighbor.

Network area

Disabled.

Nonstop Forwarding (NSF) awareness

Enabled. Allows Layer 3 switches to continue forwarding packets from a neighboring NSF-capable router during hardware or software changes.

NSF capability

Disabled.

Note 

The switch stack supports OSPF NSF-capable routing for IPv4.

Router ID

No OSPF routing process defined.

Summary address

Disabled.

Timers LSA group pacing

240 seconds.

Timers shortest path first (spf)

spf delay: 50 miliseconds; spf-holdtime: 200 miliseconds.

Virtual link

No area ID or router ID defined.

Hello interval: 10 seconds.

Retransmit interval: 5 seconds.

Transmit delay: 1 second.

Dead interval: 40 seconds.

Authentication key: no key predefined.

Message-digest key (MD5): no key predefined.

Configuring Basic OSPF Parameters

To enable OSPF, create an OSPF routing process, specify the range of IP addresses to associate with the routing process, and assign area IDs to be associated with that range. For switches running the IP services image, you can configure either the Cisco OSPFv2 NSF format or the IETF OSPFv2 NSF format.

Procedure

  Command or Action Purpose
Step 1

configure terminal

Example:


Device# configure terminal


Enters global configuration mode.

Step 2

router ospf process-id

Example:


Device(config)# router ospf 15

Enables OSPF routing, and enter router configuration mode. The process ID is an internally used identification parameter that is locally assigned and can be any positive integer. Each OSPF routing process has a unique value.

Note 

OSPF for Routed Access supports only one OSPFv2 and one OSPFv3 instance with a maximum number of 1000 dynamically learned routes.

Step 3

nsf cisco [enforce global]

Example:


Device(config)# nsf cisco enforce global

(Optional) Enables Cisco NSF operations for OSPF. The enforce global keyword cancels NSF restart when non-NSF-aware neighboring networking devices are detected.

Note 

Enter the command in Step 3 or Step 4, and go to Step 5.

Step 4

nsf ietf [restart-interval seconds]

Example:


Device(config)# nsf ietf restart-interval 60

(Optional) Enables IETF NSF operations for OSPF. The restart-interval keyword specifies the length of the graceful restart interval, in seconds. The range is from 1 to 1800. The default is 120.

Note 

Enter the command in Step 3 or Step 4, and go to Step 5.

Step 5

network address wildcard-mask area area-id

Example:


Device(config)# network 10.1.1.1 255.240.0.0 area 20

Define an interface on which OSPF runs and the area ID for that interface. You can use the wildcard-mask to use a single command to define one or more multiple interfaces to be associated with a specific OSPF area. The area ID can be a decimal value or an IP address.

Step 6

end

Example:


Device(config)# end


Returns to privileged EXEC mode.

Step 7

show ip protocols

Example:


Device# show ip protocols

Verifies your entries.

Step 8

copy running-config startup-config

Example:


Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Configuring OSPF Interfaces

You can use the ip ospf interface configuration commands to modify interface-specific OSPF parameters. You are not required to modify any of these parameters, but some interface parameters (hello interval, dead interval, and authentication key) must be consistent across all routers in an attached network. If you modify these parameters, be sure all routers in the network have compatible values.


Note

The ip ospf interface configuration commands are all optional.


Procedure

  Command or Action Purpose
Step 1

configure terminal

Example:


Device# configure terminal


Enters global configuration mode.

Step 2

interface interface-id

Example:


Device(config)# interface gigabitethernet 1/0/1

Enters interface configuration mode, and specifies the Layer 3 interface to configure.

Step 3

ip ospf cost

Example:


Device(config-if)# ip ospf 8

(Optional) Explicitly specifies the cost of sending a packet on the interface.

Step 4

ip ospf retransmit-interval seconds

Example:


Device(config-if)# ip ospf transmit-interval 10

(Optional) Specifies the number of seconds between link state advertisement transmissions. The range is 1 to 65535 seconds. The default is 5 seconds.

Step 5

ip ospf transmit-delay seconds

Example:


Device(config-if)# ip ospf transmit-delay 2

(Optional) Sets the estimated number of seconds to wait before sending a link state update packet. The range is 1 to 65535 seconds. The default is 1 second.

Step 6

ip ospf priority number

Example:


Device(config-if)# ip ospf priority 5

(Optional) Sets priority to help find the OSPF designated router for a network. The range is from 0 to 255. The default is 1.

Step 7

ip ospf hello-interval seconds

Example:


Device(config-if)# ip ospf hello-interval 12

(Optional) Sets the number of seconds between hello packets sent on an OSPF interface. The value must be the same for all nodes on a network. The range is 1 to 65535 seconds. The default is 10 seconds.

Step 8

ip ospf dead-interval seconds

Example:


Device(config-if)# ip ospf dead-interval 8

(Optional) Sets the number of seconds after the last device hello packet was seen before its neighbors declare the OSPF router to be down. The value must be the same for all nodes on a network. The range is 1 to 65535 seconds. The default is 4 times the hello interval.

Step 9

ip ospf authentication-key key

Example:


Device(config-if)# ip ospf authentication-key password

(Optional) Assign a password to be used by neighboring OSPF routers. The password can be any string of keyboard-entered characters up to 8 bytes in length. All neighboring routers on the same network must have the same password to exchange OSPF information.

Step 10

ip ospf message digest-key keyid md5 key

Example:


Device(config-if)# ip ospf message digest-key 16 md5 your1pass

(Optional) Enables MDS authentication.

  • keyid —An identifier from 1 to 255.

  • key —An alphanumeric password of up to 16 bytes.

Step 11

ip ospf database-filter all out

Example:


Device(config-if)# ip ospf database-filter all out

(Optional) Block flooding of OSPF LSA packets to the interface. By default, OSPF floods new LSAs over all interfaces in the same area, except the interface on which the LSA arrives.

Step 12

end

Example:


Device(config)# end


Returns to privileged EXEC mode.

Step 13

show ip ospf interface [interface-name]

Example:


Device# show ip ospf interface

Displays OSPF-related interface information.

Step 14

show ip ospf neighbor detail

Example:


Device# show ip ospf neighbor detail

Displays NSF awareness status of neighbor switch. The output matches one of these examples:

  • Options is 0x52

    LLS Options is 0x1 (LR)

    When both of these lines appear, the neighbor switch is NSF aware.

  • Options is 0x42 —This means the neighbor switch is not NSF aware.

Step 15

copy running-config startup-config

Example:


Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Configuring OSPF Area Parameters

Before you begin


Note

The OSPF area router configuration commands are all optional.


Procedure

  Command or Action Purpose
Step 1

configure terminal

Example:


Device# configure terminal


Enters global configuration mode.

Step 2

router ospf process-id

Example:


Device(config)# router ospf 109

Enables OSPF routing, and enter router configuration mode.

Step 3

area area-id authentication

Example:


Device(config-router)# area 1 authentication

(Optional) Allow password-based protection against unauthorized access to the identified area. The identifier can be either a decimal value or an IP address.

Step 4

area area-id authentication message-digest

Example:


Device(config-router)# area 1 authentication message-digest

(Optional) Enables MD5 authentication on the area.

Step 5

area area-id stub [no-summary]

Example:


Device(config-router)# area 1 stub

(Optional) Define an area as a stub area. The no-summary keyword prevents an ABR from sending summary link advertisements into the stub area.

Step 6

area area-id nssa [no-redistribution] [default-information-originate] [no-summary]

Example:


Device(config-router)# area 1 nssa default-information-originate

(Optional) Defines an area as a not-so-stubby-area. Every router within the same area must agree that the area is NSSA. Select one of these keywords:

  • no-redistribution —Select when the router is an NSSA ABR and you want the redistribute command to import routes into normal areas, but not into the NSSA.

  • default-information-originate —Select on an ABR to allow importing type 7 LSAs into the NSSA.

  • no-redistribution —Select to not send summary LSAs into the NSSA.

Step 7

area area-id range address mask

Example:


Device(config-router)# area 1 range 255.240.0.0

(Optional) Specifies an address range for which a single route is advertised. Use this command only with area border routers.

Step 8

end

Example:


Device(config)# end


Returns to privileged EXEC mode.

Step 9

show ip ospf [process-id]

Example:


Device# show ip ospf

Displays information about the OSPF routing process in general or for a specific process ID to verify configuration.

Step 10

show ip ospf [process-id [area-id]] database

Example:


Device# show ip osfp database

Displays lists of information related to the OSPF database for a specific router.

Step 11

copy running-config startup-config

Example:


Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Configuring Other OSPF Parameters

Procedure

  Command or Action Purpose
Step 1

configure terminal

Example:


Device# configure terminal


Enters global configuration mode.

Step 2

router ospf process-id

Example:


Device(config)# router ospf 10

Enables OSPF routing, and enter router configuration mode.

Step 3

summary-address address mask

Example:


Device(config)# summary-address 10.1.1.1 255.255.255.0

(Optional) Specifies an address and IP subnet mask for redistributed routes so that only one summary route is advertised.

Step 4

area area-id virtual-link router-id [hello-interval seconds] [retransmit-interval seconds] [trans] [[authentication-key key] | message-digest-key keyid md5 key]]

Example:


Device(config)# area 2 virtual-link 192.168.255.1 hello-interval 5

(Optional) Establishes a virtual link and set its parameters.

Step 5

default-information originate [always] [metric metric-value] [metric-type type-value] [route-map map-name]

Example:


Device(config)# default-information originate metric 100 metric-type 1

(Optional) Forces the ASBR to generate a default route into the OSPF routing domain. Parameters are all optional.

Step 6

ip ospf name-lookup

Example:


Device(config)# ip ospf name-lookup

(Optional) Configures DNS name lookup. The default is disabled.

Step 7

ip auto-cost reference-bandwidth ref-bw

Example:


Device(config)# ip auto-cost reference-bandwidth 5

(Optional) Specifies an address range for which a single route will be advertised. Use this command only with area border routers.

Step 8

distance ospf {[inter-area dist1] [inter-area dist2] [external dist3]}

Example:


Device(config)# distance ospf inter-area 150

(Optional) Changes the OSPF distance values. The default distance for each type of route is 110. The range is 1 to 255.

Step 9

passive-interface type number

Example:


Device(config)# passive-interface gigabitethernet 1/0/6

(Optional) Suppresses the sending of hello packets through the specified interface.

Step 10

timers throttle spf spf-delay spf-holdtime spf-wait

Example:


Device(config)# timers throttle spf 200 100 100

(Optional) Configures route calculation timers.

  • spf-delay —Delay between receiving a change to SPF calculation. The range is from 1 to 600000 in miliseconds.

  • spf-holdtime —Delay between first and second SPF calculation. The range is from 1 to 600000 in milliseconds.

  • spf-wait —Maximum wait time in milliseconds for SPF calculations. The range is from 1 to 600000 in milliseconds.

Step 11

ospf log-adj-changes

Example:


Device(config)# ospf log-adj-changes

(Optional) Sends syslog message when a neighbor state changes.

Step 12

end

Example:


Device(config)# end


Returns to privileged EXEC mode.

Step 13

show ip ospf [process-id [area-id]] database

Example:


Device# show ip ospf database

Displays lists of information related to the OSPF database for a specific router.

Step 14

copy running-config startup-config

Example:


Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Changing LSA Group Pacing

Procedure

  Command or Action Purpose
Step 1

configure terminal

Example:


Device# configure terminal


Enters global configuration mode.

Step 2

router ospf process-id

Example:


Device(config)# router ospf 25

Enables OSPF routing, and enter router configuration mode.

Step 3

timers lsa-group-pacing seconds

Example:


Device(config-router)# timers lsa-group-pacing 15

Changes the group pacing of LSAs.

Step 4

end

Example:


Device(config)# end


Returns to privileged EXEC mode.

Step 5

show running-config

Example:


Device# show running-config 


Verifies your entries.

Step 6

copy running-config startup-config

Example:


Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Configuring a Loopback Interface

Procedure

  Command or Action Purpose
Step 1

configure terminal

Example:


Device# configure terminal


Enters global configuration mode.

Step 2

interface loopback 0

Example:


Device(config)# interface loopback 0

Creates a loopback interface, and enter interface configuration mode.

Step 3

ip address address mask

Example:


Device(config-if)# ip address 10.1.1.5 255.255.240.0

Assign an IP address to this interface.

Step 4

end

Example:


Device(config)# end


Returns to privileged EXEC mode.

Step 5

show ip interface

Example:


Device# show ip interface

Verifies your entries.

Step 6

copy running-config startup-config

Example:


Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Monitoring OSPF

You can display specific statistics such as the contents of IP routing tables, caches, and databases.

Table 6. Show IP OSPF Statistics Commands

show ip ospf [process-id]

Displays general information about OSPF routing processes.

show ip ospf [process-id] database [router] [link-state-id]

show ip ospf [process-id] database [router] [self-originate]

show ip ospf [process-id] database [router] [adv-router [ip-address]]

show ip ospf [process-id] database [network] [link-state-id]

show ip ospf [process-id] database [summary] [link-state-id]

show ip ospf [process-id] database [asbr-summary] [link-state-id]

show ip ospf [process-id] database [external] [link-state-id]

show ip ospf [process-id area-id] database [database-summary]

Displays lists of information related to the OSPF database.

show ip ospf border-routes

Displays the internal OSPF routing ABR and ASBR table entries.

show ip ospf interface [interface-name]

Displays OSPF-related interface information.

show ip ospf neighbor [interface-name] [neighbor-id] detail

Displays OSPF interface neighbor information.

show ip ospf virtual-links

Displays OSPF-related virtual links information.

Configuration Examples for OSPF

Example: Configuring Basic OSPF Parameters

This example shows how to configure an OSPF routing process and assign it a process number of 109:


Device(config)# router ospf 109
Device(config-router)# network 131.108.0.0 255.255.255.0 area 24

Information About EIGRP

Enhanced IGRP (EIGRP) is a Cisco proprietary enhanced version of the IGRP. EIGRP uses the same distance vector algorithm and distance information as IGRP; however, the convergence properties and the operating efficiency of EIGRP are significantly improved.

The convergence technology employs an algorithm referred to as the Diffusing Update Algorithm (DUAL), which guarantees loop-free operation at every instant throughout a route computation and allows all devices involved in a topology change to synchronize at the same time. Routers that are not affected by topology changes are not involved in recomputations.

IP EIGRP provides increased network width. With RIP, the largest possible width of your network is 15 hops. Because the EIGRP metric is large enough to support thousands of hops, the only barrier to expanding the network is the transport-layer hop counter. EIGRP increments the transport control field only when an IP packet has traversed 15 routers and the next hop to the destination was learned through EIGRP. When a RIP route is used as the next hop to the destination, the transport control field is incremented as usual.

EIGRP Features

EIGRP offers these features:

  • Fast convergence.

  • Incremental updates when the state of a destination changes, instead of sending the entire contents of the routing table, minimizing the bandwidth required for EIGRP packets.

  • Less CPU usage because full update packets need not be processed each time they are received.

  • Protocol-independent neighbor discovery mechanism to learn about neighboring routers.

  • Variable-length subnet masks (VLSMs).

  • Arbitrary route summarization.

  • EIGRP scales to large networks.

EIGRP Components

EIGRP has these four basic components:

  • Neighbor discovery and recovery is the process that routers use to dynamically learn of other routers on their directly attached networks. Routers must also discover when their neighbors become unreachable or inoperative. Neighbor discovery and recovery is achieved with low overhead by periodically sending small hello packets. As long as hello packets are received, the Cisco IOS software can learn that a neighbor is alive and functioning. When this status is determined, the neighboring routers can exchange routing information.

  • The reliable transport protocol is responsible for guaranteed, ordered delivery of EIGRP packets to all neighbors. It supports intermixed transmission of multicast and unicast packets. Some EIGRP packets must be sent reliably, and others need not be. For efficiency, reliability is provided only when necessary. For example, on a multiaccess network that has multicast capabilities (such as Ethernet), it is not necessary to send hellos reliably to all neighbors individually. Therefore, EIGRP sends a single multicast hello with an indication in the packet informing the receivers that the packet need not be acknowledged. Other types of packets (such as updates) require acknowledgment, which is shown in the packet. The reliable transport has a provision to send multicast packets quickly when there are unacknowledged packets pending. Doing so helps ensure that convergence time remains low in the presence of varying speed links.

  • The DUAL finite state machine embodies the decision process for all route computations. It tracks all routes advertised by all neighbors. DUAL uses the distance information (known as a metric) to select efficient, loop-free paths. DUAL selects routes to be inserted into a routing table based on feasible successors. A successor is a neighboring router used for packet forwarding that has a least-cost path to a destination that is guaranteed not to be part of a routing loop. When there are no feasible successors, but there are neighbors advertising the destination, a recomputation must occur. This is the process whereby a new successor is determined. The amount of time it takes to recompute the route affects the convergence time. Recomputation is processor-intensive; it is advantageous to avoid recomputation if it is not necessary. When a topology change occurs, DUAL tests for feasible successors. If there are feasible successors, it uses any it finds to avoid unnecessary recomputation.

  • The protocol-dependent modules are responsible for network layer protocol-specific tasks. An example is the IP EIGRP module, which is responsible for sending and receiving EIGRP packets that are encapsulated in IP. It is also responsible for parsing EIGRP packets and informing DUAL of the new information received. EIGRP asks DUAL to make routing decisions, but the results are stored in the IP routing table. EIGRP is also responsible for redistributing routes learned by other IP routing protocols.


    Note

    To enable EIGRP, the Device or stack master must be running the


EIGRP Nonstop Forwarding

The Device stack supports two levels of EIGRP nonstop forwarding:

  • EIGRP NSF Awareness

  • EIGRP NSF Capability

EIGRP NSF Awareness

The supports EIGRP NSF Awareness for IPv4. When the neighboring router is NSF-capable, the Layer 3 Device continues to forward packets from the neighboring router during the interval between the primary Route Processor (RP) in a router failing and the backup RP taking over, or while the primary RP is manually reloaded for a nondisruptive software upgrade.

This feature cannot be disabled. For more information on this feature, see the “EIGRP Nonstop Forwarding (NSF) Awareness” section of the Cisco IOS IP Routing Protocols Configuration Guide, Release 12.4.

EIGRP NSF Capability

The supports EIGRP Cisco NSF routing to speed up convergence and to eliminate traffic loss after a stack master change. For details about this NSF capability, see the “Configuring Nonstop Forwarding” chapter in the High Availability Configuration Guide, Cisco IOS XE Release 3S.

The also supports EIGRP NSF-capable routing for IPv4 for better convergence and lower traffic loss following a stack master change. When an EIGRP NSF-capable stack master restarts or a new stack master starts up and NSF restarts, the Device has no neighbors, and the topology table is empty. The Device must bring up the interfaces, reacquire neighbors, and rebuild the topology and routing tables without interrupting the traffic directed toward the Device stack. EIGRP peer routers maintain the routes learned from the new stack master and continue forwarding traffic through the NSF restart process.

To prevent an adjacency reset by the neighbors, the new stack master uses a new Restart (RS) bit in the EIGRP packet header to show the restart. When the neighbor receives this, it synchronizes the stack in its peer list and maintains the adjacency with the stack. The neighbor then sends its topology table to the stack master with the RS bit set to show that it is NSF-aware and is aiding the new stack master.

If at least one of the stack peer neighbors is NSF-aware, the stack master receives updates and rebuilds its database. Each NSF-aware neighbor sends an end of table (EOT) marker in the last update packet to mark the end of the table content. The stack master recognizes the convergence when it receives the EOT marker, and it then begins sending updates. When the stack master has received all EOT markers from its neighbors or when the NSF converge timer expires, EIGRP notifies the routing information database (RIB) of convergence and floods its topology table to all NSF-aware peers.

EIGRP Stub Routing

The EIGRP stub routing feature, available in all feature sets, reduces resource utilization by moving routed traffic closer to the end user.


Note

The IP Base feature set contains EIGRP stub routing capability, which only advertises connected or summary routes from the routing tables to other device in the network. The device uses EIGRP stub routing at the access layer to eliminate the need for other types of routing advertisements. For enhanced capability and complete EIGRP routing, the device must be running the IP Base feature set. On a device running the IP base feature set, if you try to configure multi-VRF-CE and EIGRP stub routing at the same time, the configuration is not allowed. IPv6 EIGRP stub routing is not supported with the IP base feature set.


In a network using EIGRP stub routing, the only allowable route for IP traffic to the user is through a device that is configured with EIGRP stub routing. The device sends the routed traffic to interfaces that are configured as user interfaces or are connected to other devices.

When using EIGRP stub routing, you need to configure the distribution and remote routers to use EIGRP and to configure only the device as a stub. Only specified routes are propagated from the device. The device responds to all queries for summaries, connected routes, and routing updates.

Any neighbor that receives a packet informing it of the stub status does not query the stub router for any routes, and a router that has a stub peer does not query that peer. The stub router depends on the distribution router to send the proper updates to all peers.

In the figure given below, device B is configured as an EIGRP stub router. Devicees A and C are connected to the rest of the WAN. Device B advertises connected, static, redistribution, and summary routes to Device A and C. Device B does not advertise any routes learned from Device A (and the reverse).

Figure 4. EIGRP Stub Router Configuration

For more information about EIGRP stub routing, see “Configuring EIGRP Stub Routing” section of the Cisco IOS IP Configuration Guide, Volume 2 of 3: Routing Protocols.

How to Configure EIGRP

To create an EIGRP routing process, you must enable EIGRP and associate networks. EIGRP sends updates to the interfaces in the specified networks. If you do not specify an interface network, it is not advertised in any EIGRP update.


Note

If you have routers on your network that are configured for IGRP, and you want to change to EIGRP, you must designate transition routers that have both IGRP and EIGRP configured. In these cases, perform Steps 1 through 3 in the next section and also see the “Configuring Split Horizon” section. You must use the same AS number for routes to be automatically redistributed.


Default EIGRP Configuration

Table 7. Default EIGRP Configuration

Feature

Default Setting

Auto summary

Disabled.

Default-information

Exterior routes are accepted and default information is passed between EIGRP processes when doing redistribution.

Default metric

Only connected routes and interface static routes can be redistributed without a default metric. The metric includes:

  • Bandwidth: 0 or greater kb/s.

  • Delay (tens of microseconds): 0 or any positive number that is a multiple of 39.1 nanoseconds.

  • Reliability: any number between 0 and 255 (255 means 100 percent reliability).

  • Loading: effective bandwidth as a number between 0 and 255 (255 is 100 percent loading).

  • MTU: maximum transmission unit size of the route in bytes. 0 or any positive integer.

Distance

Internal distance: 90.

External distance: 170.

EIGRP log-neighbor changes

Disabled. No adjacency changes logged.

IP authentication key-chain

No authentication provided.

IP authentication mode

No authentication provided.

IP bandwidth-percent

50 percent.

IP hello interval

For low-speed nonbroadcast multiaccess (NBMA) networks: 60 seconds; all other networks: 5 seconds.

IP hold-time

For low-speed NBMA networks: 180 seconds; all other networks: 15 seconds.

IP split-horizon

Enabled.

IP summary address

No summary aggregate addresses are predefined.

Metric weights

tos: 0; k1 and k3: 1; k2, k4, and k5: 0

Network

None specified.

Nonstop Forwarding (NSF) Awareness

Enabled for IPv4 on switches running the Allows Layer 3 switches to continue forwarding packets from a neighboring NSF-capable router during hardware or software changes.

NSF capability

Disabled.

Note 

The Device supports EIGRP NSF-capable routing for IPv4.

Offset-list

Disabled.

Router EIGRP

Disabled.

Set metric

No metric set in the route map.

Traffic-share

Distributed proportionately to the ratios of the metrics.

Variance

1 (equal-cost load-balancing).

Configuring Basic EIGRP Parameters

Procedure

  Command or Action Purpose
Step 1

configure terminal

Example:


Device# configure terminal


Enters global configuration mode.

Step 2

router eigrp autonomous-system

Example:


Device(config)# router eigrp 10

Enables an EIGRP routing process, and enter router configuration mode. The AS number identifies the routes to other EIGRP routers and is used to tag routing information.

Step 3

nsf

Example:


Device(config)# nsf

(Optional) Enables EIGRP NSF. Enter this command on the stack master and on all of its peers.

Step 4

network network-number

Example:


Device(config)# network 192.168.0.0

Associate networks with an EIGRP routing process. EIGRP sends updates to the interfaces in the specified networks.

Step 5

eigrp log-neighbor-changes

Example:


Device(config)# eigrp log-neighbor-changes

(Optional) Enables logging of EIGRP neighbor changes to monitor routing system stability.

Step 6

metric weights tos k1 k2 k3 k4 k5

Example:


Device(config)# metric weights 0 2 0 2 0 0

(Optional) Adjust the EIGRP metric. Although the defaults have been carefully set to provide excellent operation in most networks, you can adjust them.

Caution 

Setting metrics is complex and is not recommended without guidance from an experienced network designer.

Step 7

offset-list [access-list number | name] {in | out} offset [type number]

Example:


Device(config)# offset-list 21 out 10

(Optional) Applies an offset list to routing metrics to increase incoming and outgoing metrics to routes learned through EIGRP. You can limit the offset list with an access list or an interface.

Step 8

auto-summary

Example:


Device(config)# auto-summary

(Optional) Enables automatic summarization of subnet routes into network-level routes.

Step 9

ip summary-address eigrp autonomous-system-number address mask

Example:


Device(config)# ip summary-address eigrp 1 192.168.0.0 255.255.0.0

(Optional) Configures a summary aggregate.

Step 10

end

Example:


Device(config)# end


Returns to privileged EXEC mode.

Step 11

show ip protocols

Example:


Device# show ip protocols

Verifies your entries.

For NSF awareness, the output shows:

*** IP Routing is NSF aware *** EIGRP NSF enabled

Step 12

copy running-config startup-config

Example:


Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Configuring EIGRP Interfaces

Other optional EIGRP parameters can be configured on an interface basis.

Procedure

  Command or Action Purpose
Step 1

configure terminal

Example:


Device# configure terminal


Enters global configuration mode.

Step 2

interface interface-id

Example:


Device(config)# interface gigabitethernet 1/0/1

Enters interface configuration mode, and specifies the Layer 3 interface to configure.

Step 3

ip bandwidth-percent eigrp percent

Example:


Device(config-if)# ip bandwidth-percent eigrp 60

(Optional) Configures the percentage of bandwidth that can be used by EIGRP on an interface. The default is 50 percent.

Step 4

ip summary-address eigrp autonomous-system-number address mask

Example:


Device(config-if)# ip summary-address eigrp 109 192.161.0.0 255.255.0.0

(Optional) Configures a summary aggregate address for a specified interface (not usually necessary if auto-summary is enabled).

Step 5

ip hello-interval eigrp autonomous-system-number seconds

Example:


Device(config-if)# ip hello-interval eigrp 109 10

(Optional) Change the hello time interval for an EIGRP routing process. The range is 1 to 65535 seconds. The default is 60 seconds for low-speed NBMA networks and 5 seconds for all other networks.

Step 6

ip hold-time eigrp autonomous-system-number seconds

Example:


Device(config-if)# ip hold-time eigrp 109 40

(Optional) Change the hold time interval for an EIGRP routing process. The range is 1 to 65535 seconds. The default is 180 seconds for low-speed NBMA networks and 15 seconds for all other networks.

Caution 

Do not adjust the hold time without consulting Cisco technical support.

Step 7

no ip split-horizon eigrp autonomous-system-number

Example:


Device(config-if)# no ip split-horizon eigrp 109

(Optional) Disables split horizon to allow route information to be advertised by a router out any interface from which that information originated.

Step 8

end

Example:


Device(config)# end


Returns to privileged EXEC mode.

Step 9

show ip eigrp interface

Example:


Device# show ip eigrp interface

Displays which interfaces EIGRP is active on and information about EIGRP relating to those interfaces.

Step 10

copy running-config startup-config

Example:


Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Configuring EIGRP Route Authentication

EIGRP route authentication provides MD5 authentication of routing updates from the EIGRP routing protocol to prevent the introduction of unauthorized or false routing messages from unapproved sources.

Procedure

  Command or Action Purpose
Step 1

configure terminal

Example:


Device# configure terminal


Enters global configuration mode.

Step 2

interface interface-id

Example:


Device(config)# interface gigabitethernet 1/0/1

Enters interface configuration mode, and specifies the Layer 3 interface to configure.

Step 3

ip authentication mode eigrp autonomous-system md5

Example:


Device(config-if)# ip authentication mode eigrp 104 md5

Enables MD5 authentication in IP EIGRP packets.

Step 4

ip authentication key-chain eigrp autonomous-system key-chain

Example:


Device(config-if)# ip authentication key-chain eigrp 105 chain1

Enables authentication of IP EIGRP packets.

Step 5

exit

Example:


Device(config-if)# exit

Returns to global configuration mode.

Step 6

key chain name-of-chain

Example:


Device(config)# key chain chain1

Identify a key chain and enter key-chain configuration mode. Match the name configured in Step 4.

Step 7

key number

Example:


Device(config-keychain)# key 1

In key-chain configuration mode, identify the key number.

Step 8

key-string text

Example:


Device(config-keychain-key)# key-string key1

In key-chain key configuration mode, identify the key string.

Step 9

accept-lifetime start-time {infinite | end-time | duration seconds}

Example:


Device(config-keychain-key)# accept-lifetime 13:30:00 Jan 25 2011 duration 7200

(Optional) Specifies the time period during which the key can be received.

The start-time and end-time syntax can be either hh:mm:ss Month date year or hh:mm:ss date Month year . The default is forever with the default start-time and the earliest acceptable date as January 1, 1993. The default end-time and duration is infinite .

Step 10

send-lifetime start-time {infinite | end-time | duration seconds}

Example:


Device(config-keychain-key)# send-lifetime 14:00:00 Jan 25 2011 duration 3600

(Optional) Specifies the time period during which the key can be sent.

The start-time and end-time syntax can be either hh:mm:ss Month date year or hh:mm:ss date Month year . The default is forever with the default start-time and the earliest acceptable date as January 1, 1993. The default end-time and duration is infinite .

Step 11

end

Example:


Device(config)# end


Returns to privileged EXEC mode.

Step 12

show key chain

Example:


Device# show key chain

Displays authentication key information.

Step 13

copy running-config startup-config

Example:


Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Monitoring and Maintaining EIGRP

You can delete neighbors from the neighbor table. You can also display various EIGRP routing statistics. The table given below lists the privileged EXEC commands for deleting neighbors and displaying statistics. For explanations of fields in the resulting display, see the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.4.

Table 8. IP EIGRP Clear and Show Commands

clear ip eigrp neighbors [if-address | interface]

Deletes neighbors from the neighbor table.

show ip eigrp interface [interface] [as number]

Displays information about interfaces configured for EIGRP.

show ip eigrp neighbors [type-number]

Displays EIGRP discovered neighbors.

show ip eigrp topology [autonomous-system-number] | [[ip-address] mask]]

Displays the EIGRP topology table for a given process.

show ip eigrp traffic [autonomous-system-number]

Displays the number of packets sent and received for all or a specified EIGRP process.

Information About BGP

The Border Gateway Protocol (BGP) is an exterior gateway protocol used to set up an interdomain routing system that guarantees the loop-free exchange of routing information between autonomous systems. Autonomous systems are made up of routers that operate under the same administration and that run Interior Gateway Protocols (IGPs), such as RIP or OSPF, within their boundaries and that interconnect by using an Exterior Gateway Protocol (EGP). BGP Version 4 is the standard EGP for interdomain routing in the Internet. The protocol is defined in RFCs 1163, 1267, and 1771. You can find detailed information about BGP in Internet Routing Architectures, published by Cisco Press, and in the “Configuring BGP” chapter in the Cisco IP and IP Routing Configuration Guide.

For details about BGP commands and keywords, see the “IP Routing Protocols” part of the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols .

BGP Network Topology

Routers that belong to the same autonomous system (AS) and that exchange BGP updates run internal BGP (IBGP), and routers that belong to different autonomous systems and that exchange BGP updates run external BGP (EBGP). Most configuration commands are the same for configuring EBGP and IBGP. The difference is that the routing updates are exchanged either between autonomous systems (EBGP) or within an AS (IBGP). The figure given below shows a network that is running both EBGP and IBGP.

Figure 5. EBGP, IBGP, and Multiple Autonomous Systems

Before exchanging information with an external AS, BGP ensures that networks within the AS can be reached by defining internal BGP peering among routers within the AS and by redistributing BGP routing information to IGPs that run within the AS, such as IGRP and OSPF.

Routers that run a BGP routing process are often referred to as BGP speakers. BGP uses the Transmission Control Protocol (TCP) as its transport protocol (specifically port 179). Two BGP speakers that have a TCP connection to each other for exchanging routing information are known as peers or neighbors. In the above figure, Routers A and B are BGP peers, as are Routers B and C and Routers C and D. The routing information is a series of AS numbers that describe the full path to the destination network. BGP uses this information to construct a loop-free map of autonomous systems.

The network has these characteristics:

  • Routers A and B are running EBGP, and Routers B and C are running IBGP. Note that the EBGP peers are directly connected and that the IBGP peers are not. As long as there is an IGP running that allows the two neighbors to reach one another, IBGP peers do not have to be directly connected.

  • All BGP speakers within an AS must establish a peer relationship with each other. That is, the BGP speakers within an AS must be fully meshed logically. BGP4 provides two techniques that reduce the requirement for a logical full mesh: confederations and route reflectors.

  • AS 200 is a transit AS for AS 100 and AS 300—that is, AS 200 is used to transfer packets between AS 100 and AS 300.

BGP peers initially exchange their full BGP routing tables and then send only incremental updates. BGP peers also exchange keepalive messages (to ensure that the connection is up) and notification messages (in response to errors or special conditions).

In BGP, each route consists of a network number, a list of autonomous systems that information has passed through (the autonomous system path), and a list of other path attributes. The primary function of a BGP system is to exchange network reachability information, including information about the list of AS paths, with other BGP systems. This information can be used to determine AS connectivity, to prune routing loops, and to enforce AS-level policy decisions.

A router or Device running Cisco IOS does not select or use an IBGP route unless it has a route available to the next-hop router and it has received synchronization from an IGP (unless IGP synchronization is disabled). When multiple routes are available, BGP bases its path selection on attribute values. See the “Configuring BGP Decision Attributes” section for information about BGP attributes.

BGP Version 4 supports classless interdomain routing (CIDR) so you can reduce the size of your routing tables by creating aggregate routes, resulting in supernets. CIDR eliminates the concept of network classes within BGP and supports the advertising of IP prefixes.

Nonstop Forwarding Awareness

The BGP NSF Awareness feature is supported for IPv4 in the . To enable this feature with BGP routing, you need to enable Graceful Restart. When the neighboring router is NSF-capable, and this feature is enabled, the Layer 3 Device continues to forward packets from the neighboring router during the interval between the primary Route Processor (RP) in a router failing and the backup RP taking over, or while the primary RP is manually reloaded for a nondisruptive software upgrade.

For more information, see the “BGP Nonstop Forwarding (NSF) Awareness” section of the Cisco IOS IP Routing Protocols Configuration Guide, Release 12.4.

Information About BGP Routing

To enable BGP routing, you establish a BGP routing process and define the local network. Because BGP must completely recognize the relationships with its neighbors, you must also specify a BGP neighbor.

BGP supports two kinds of neighbors: internal and external. Internal neighbors are in the same AS; external neighbors are in different autonomous systems. External neighbors are usually adjacent to each other and share a subnet, but internal neighbors can be anywhere in the same AS.

The switch supports the use of private AS numbers, usually assigned by service providers and given to systems whose routes are not advertised to external neighbors. The private AS numbers are from 64512 to 65535. You can configure external neighbors to remove private AS numbers from the AS path by using the neighbor remove-private-as router configuration command. Then when an update is passed to an external neighbor, if the AS path includes private AS numbers, these numbers are dropped.

If your AS will be passing traffic through it from another AS to a third AS, it is important to be consistent about the routes it advertises. If BGP advertised a route before all routers in the network had learned about the route through the IGP, the AS might receive traffic that some routers could not yet route. To prevent this from happening, BGP must wait until the IGP has propagated information across the AS so that BGP is synchronized with the IGP. Synchronization is enabled by default. If your AS does not pass traffic from one AS to another AS, or if all routers in your autonomous systems are running BGP, you can disable synchronization, which allows your network to carry fewer routes in the IGP and allows BGP to converge more quickly.

Routing Policy Changes

Routing policies for a peer include all the configurations that might affect inbound or outbound routing table updates. When you have defined two routers as BGP neighbors, they form a BGP connection and exchange routing information. If you later change a BGP filter, weight, distance, version, or timer, or make a similar configuration change, you must reset the BGP sessions so that the configuration changes take effect.

There are two types of reset, hard reset and soft reset. Cisco IOS Releases 12.1 and later support a soft reset without any prior configuration. To use a soft reset without preconfiguration, both BGP peers must support the soft route refresh capability, which is advertised in the OPEN message sent when the peers establish a TCP session. A soft reset allows the dynamic exchange of route refresh requests and routing information between BGP routers and the subsequent re-advertisement of the respective outbound routing table.

  • When soft reset generates inbound updates from a neighbor, it is called dynamic inbound soft reset.

  • When soft reset sends a set of updates to a neighbor, it is called outbound soft reset.

A soft inbound reset causes the new inbound policy to take effect. A soft outbound reset causes the new local outbound policy to take effect without resetting the BGP session. As a new set of updates is sent during outbound policy reset, a new inbound policy can also take effect.

The table given below lists the advantages and disadvantages hard reset and soft reset.

Table 9. Advantages and Disadvantages of Hard and Soft Resets

Type of Reset

Advantages

Disadvantages

Hard reset

No memory overhead

The prefixes in the BGP, IP, and FIB tables provided by the neighbor are lost. Not recommended.

Outbound soft reset

No configuration, no storing of routing table updates

Does not reset inbound routing table updates.

Dynamic inbound soft reset

Does not clear the BGP session and cache

Does not require storing of routing table updates and has no memory overhead

Both BGP routers must support the route refresh capability (in Cisco IOS Release 12.1 and later).

BGP Decision Attributes

When a BGP speaker receives updates from multiple autonomous systems that describe different paths to the same destination, it must choose the single best path for reaching that destination. When chosen, the selected path is entered into the BGP routing table and propagated to its neighbors. The decision is based on the value of attributes that the update contains and other BGP-configurable factors.

When a BGP peer learns two EBGP paths for a prefix from a neighboring AS, it chooses the best path and inserts that path in the IP routing table. If BGP multipath support is enabled and the EBGP paths are learned from the same neighboring autonomous systems, instead of a single best path, multiple paths are installed in the IP routing table. Then, during packet switching, per-packet or per-destination 
load-balancing is performed among the multiple paths. The maximum-paths router configuration command controls the number of paths allowed.

These factors summarize the order in which BGP evaluates the attributes for choosing the best path:

  1. If the path specifies a next hop that is inaccessible, drop the update. The BGP next-hop attribute, automatically determined by the software, is the IP address of the next hop that is going to be used to reach a destination. For EBGP, this is usually the IP address of the neighbor specified by the neighbor remote-as router configuration command. You can disable next-hop processing by using route maps or the neighbor next-hop-self router configuration command.

  2. Prefer the path with the largest weight (a Cisco proprietary parameter). The weight attribute is local to the router and not propagated in routing updates. By default, the weight attribute is 32768 for paths that the router originates and zero for other paths. Routes with the largest weight are preferred. You can use access lists, route maps, or the neighbor weight router configuration command to set weights.

  3. Prefer the route with the highest local preference. Local preference is part of the routing update and exchanged among routers in the same AS. The default value of the local preference attribute is 100. You can set local preference by using the bgp default local-preference router configuration command or by using a route map.

  4. Prefer the route that was originated by BGP running on the local router.

  5. Prefer the route with the shortest AS path.

  6. Prefer the route with the lowest origin type. An interior route or IGP is lower than a route learned by EGP, and an EGP-learned route is lower than one of unknown origin or learned in another way.

  7. Prefer the route with the lowest multi -exit discriminator (MED) metric attribute if the neighboring AS is the same for all routes considered. You can configure the MED by using route maps or by using the default-metric router configuration command. When an update is sent to an IBGP peer, the MED is included.

  8. Prefer the external (EBGP) path over the internal (IBGP) path.

  9. Prefer the route that can be reached through the closest IGP neighbor (the lowest IGP metric). This means that the router will prefer the shortest internal path within the AS to reach the destination (the shortest path to the BGP next-hop).

  10. If the following conditions are all true, insert the route for this path into the IP routing table:

    Both the best route and this route are external.

    Both the best route and this route are from the same neighboring autonomous system.

    Maximum-paths is enabled.

  11. If multipath is not enabled, prefer the route with the lowest IP address value for the BGP router ID. The router ID is usually the highest IP address on the router or the loopback (virtual) address, but might be implementation-specific.

Route Maps

Within BGP, route maps can be used to control and to modify routing information and to define the conditions by which routes are redistributed between routing domains. See the “Using Route Maps to Redistribute Routing Information” section for more information about route maps. Each route map has a name that identifies the route map (map tag ) and an optional sequence number.

BGP Filtering

You can filter BGP advertisements by using AS-path filters, such as the as-path access-list global configuration command and the neighbor filter-list router configuration command. You can also use access lists with the neighbor distribute-list router configuration command. Distribute-list filters are applied to network numbers. See the “Controlling Advertising and Processing in Routing Updates” section for information about the distribute-list command.

You can use route maps on a per-neighbor basis to filter updates and to modify various attributes. A route map can be applied to either inbound or outbound updates. Only the routes that pass the route map are sent or accepted in updates. On both inbound and outbound updates, matching is supported based on AS path, community, and network numbers. Autonomous system path matching requires the match as-path access-list route-map command, community based matching requires the match community-list route-map command, and network-based matching requires the ip access-list global configuration command.

Prefix List for BGP Filtering

You can use prefix lists as an alternative to access lists in many BGP route filtering commands, including the neighbor distribute-list router configuration command. The advantages of using prefix lists include performance improvements in loading and lookup of large lists, incremental update support, easier CLI configuration, and greater flexibility.

Filtering by a prefix list involves matching the prefixes of routes with those listed in the prefix list, as when matching access lists. When there is a match, the route is used. Whether a prefix is permitted or denied is based upon these rules:

  • An empty prefix list permits all prefixes.

  • An implicit deny is assumed if a given prefix does not match any entries in a prefix list.

  • When multiple entries of a prefix list match a given prefix, the sequence number of a prefix list entry identifies the entry with the lowest sequence number.

By default, sequence numbers are generated automatically and incremented in units of five. If you disable the automatic generation of sequence numbers, you must specify the sequence number for each entry. You can specify sequence values in any increment. If you specify increments of one, you cannot insert additional entries into the list; if you choose very large increments, you might run out of values.

BGP Community Filtering

One way that BGP controls the distribution of routing information based on the value of the COMMUNITIES attribute. The attribute is a way to groups destinations into communities and to apply routing decisions based on the communities. This method simplifies configuration of a BGP speaker to control distribution of routing information.

A community is a group of destinations that share some common attribute. Each destination can belong to multiple communities. AS administrators can define to which communities a destination belongs. By default, all destinations belong to the general Internet community. The community is identified by the COMMUNITIES attribute, an optional, transitive, global attribute in the numerical range from 1 to 4294967200. These are some predefined, well-known communities:

  • internet —Advertise this route to the Internet community. All routers belong to it.

  • no-export —Do not advertise this route to EBGP peers.

  • no-advertise —Do not advertise this route to any peer (internal or external).

  • local-as —Do not advertise this route to peers outside the local autonomous system.

Based on the community, you can control which routing information to accept, prefer, or distribute to other neighbors. A BGP speaker can set, append, or modify the community of a route when learning, advertising, or redistributing routes. When routes are aggregated, the resulting aggregate has a COMMUNITIES attribute that contains all communities from all the initial routes.

You can use community lists to create groups of communities to use in a match clause of a route map. As with an access list, a series of community lists can be created. Statements are checked until a match is found. As soon as one statement is satisfied, the test is concluded.

To set the COMMUNITIES attribute and match clauses based on communities, see the match community-list and set community route-map configuration commands in the “Using Route Maps to Redistribute Routing Information” section.

BGP Neighbors and Peer Groups

Often many BGP neighbors are configured with the same update policies (that is, the same outbound route maps, distribute lists, filter lists, update source, and so on). Neighbors with the same update policies can be grouped into peer groups to simplify configuration and to make updating more efficient. When you have configured many peers, we recommend this approach.

To configure a BGP peer group, you create the peer group, assign options to the peer group, and add neighbors as peer group members. You configure the peer group by using the neighbor router configuration commands. By default, peer group members inherit all the configuration options of the peer group, including the remote-as (if configured), version, update-source, out-route-map, out-filter-list, out-dist-list, minimum-advertisement-interval, and next-hop-self. All peer group members also inherit changes made to the peer group. Members can also be configured to override the options that do not affect outbound updates.

Aggregate Routes

Classless interdomain routing (CIDR) enables you to create aggregate routes (or supernets) to minimize the size of routing tables. You can configure aggregate routes in BGP either by redistributing an aggregate route into BGP or by creating an aggregate entry in the BGP routing table. An aggregate address is added to the BGP table when there is at least one more specific entry in the BGP table.

Routing Domain Confederations

One way to reduce the IBGP mesh is to divide an autonomous system into multiple subautonomous systems and to group them into a single confederation that appears as a single autonomous system. Each autonomous system is fully meshed within itself and has a few connections to other autonomous systems in the same confederation. Even though the peers in different autonomous systems have EBGP sessions, they exchange routing information as if they were IBGP peers. Specifically, the next hop, MED, and local preference information is preserved. You can then use a single IGP for all of the autonomous systems.

BGP Route Reflectors

BGP requires that all of the IBGP speakers be fully meshed. When a router receives a route from an external neighbor, it must advertise it to all internal neighbors. To prevent a routing information loop, all IBPG speakers must be connected. The internal neighbors do not send routes learned from internal neighbors to other internal neighbors.

With route reflectors, all IBGP speakers need not be fully meshed because another method is used to pass learned routes to neighbors. When you configure an internal BGP peer to be a route reflector, it is responsible for passing IBGP learned routes to a set of IBGP neighbors. The internal peers of the route reflector are divided into two groups: client peers and nonclient peers (all the other routers in the autonomous system). A route reflector reflects routes between these two groups. The route reflector and its client peers form a cluster. The nonclient peers must be fully meshed with each other, but the client peers need not be fully meshed. The clients in the cluster do not communicate with IBGP speakers outside their cluster.

When the route reflector receives an advertised route, it takes one of these actions, depending on the neighbor:

  • A route from an external BGP speaker is advertised to all clients and nonclient peers.

  • A route from a nonclient peer is advertised to all clients.

  • A route from a client is advertised to all clients and nonclient peers. Hence, the clients need not be fully meshed.

Usually a cluster of clients have a single route reflector, and the cluster is identified by the route reflector router ID. To increase redundancy and to avoid a single point of failure, a cluster might have more than one route reflector. In this case, all route reflectors in the cluster must be configured with the same 4-byte cluster ID so that a route reflector can recognize updates from route reflectors in the same cluster. All the route reflectors serving a cluster should be fully meshed and should have identical sets of client and nonclient peers.

Route Dampening

Route flap dampening is a BGP feature designed to minimize the propagation of flapping routes across an internetwork. A route is considered to be flapping when it is repeatedly available, then unavailable, then available, then unavailable, and so on. When route dampening is enabled, a numeric penalty value is assigned to a route when it flaps. When a route’s accumulated penalties reach a configurable limit, BGP suppresses advertisements of the route, even if the route is running. The reuse limit is a configurable value that is compared with the penalty. If the penalty is less than the reuse limit, a suppressed route that is up is advertised again.

Dampening is not applied to routes that are learned by IBGP. This policy prevents the IBGP peers from having a higher penalty for routes external to the AS.

More BGP Information

For detailed descriptions of BGP configuration, see the “Configuring BGP” chapter in the “IP Routing Protocols” part of the Cisco IOS IP Configuration Guide, Release 12.4. For details about specific commands, see the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.4.

How to Configure BGP

Default BGP Configuration

The table given below shows the basic default BGP configuration. For the defaults for all characteristics, see the specific commands in the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.4.

Table 10. Default BGP Configuration

Feature

Default Setting

Aggregate address

Disabled: None defined.

AS path access list

None defined.

Auto summary

Disabled.

Best path

  • The router considers as-path in choosing a route and does not compare similar routes from external BGP peers.

  • Compare router ID: Disabled.

BGP community list

  • Number: None defined. When you permit a value for the community number, the list defaults to an implicit deny for everything else that has not been permitted.

  • Format: Cisco default format (32-bit number).

BGP confederation identifier/peers

  • Identifier: None configured.

  • Peers: None identified.

BGP Fast external fallover

Enabled.

BGP local preference

100. The range is 0 to 4294967295 with the higher value preferred.

BGP network

None specified; no backdoor route advertised.

BGP route dampening

Disabled by default. When enabled:

  • Half-life is 15 minutes.

  • Re-use is 750 (10-second increments).

  • Suppress is 2000 (10-second increments).

  • Max-suppress-time is 4 times half-life; 60 minutes.

BGP router ID

The IP address of a loopback interface if one is configured or the highest IP address configured for a physical interface on the router.

Default information originate (protocol or network redistribution)

Disabled.

Default metric

Built-in, automatic metric translations.

Distance

  • External route administrative distance: 20 (acceptable values are from 1 to 255).

  • Internal route administrative distance: 200 (acceptable values are from 1 to 255).

  • Local route administrative distance: 200 (acceptable values are from 1 to 255).

Distribute list

  • In (filter networks received in updates): Disabled.

  • Out (suppress networks from being advertised in updates): Disabled.

Internal route redistribution

Disabled.

IP prefix list

None defined.

Multi exit discriminator (MED)

  • Always compare: Disabled. Does not compare MEDs for paths from neighbors in different autonomous systems.

  • Best path compare: Disabled.

  • MED missing as worst path: Disabled.

  • Deterministic MED comparison is disabled.

Neighbor

  • Advertisement interval: 30 seconds for external peers; 5 seconds for internal peers.

  • Change logging: Enabled.

  • Conditional advertisement: Disabled.

  • Default originate: No default route is sent to the neighbor.

  • Description: None.

  • Distribute list: None defined.

  • External BGP multihop: Only directly connected neighbors are allowed.

  • Filter list: None used.

  • Maximum number of prefixes received: No limit.

  • Next hop (router as next hop for BGP neighbor): Disabled.

  • Password: Disabled.

  • Peer group: None defined; no members assigned.

  • Prefix list: None specified.

  • Remote AS (add entry to neighbor BGP table): No peers defined.

  • Private AS number removal: Disabled.

  • Route maps: None applied to a peer.

  • Send community attributes: None sent to neighbors.

  • Shutdown or soft reconfiguration: Not enabled.

  • Timers: keepalive: 60 seconds; holdtime: 180 seconds.

  • Update source: Best local address.

  • Version: BGP Version 4.

  • Weight: Routes learned through BGP peer: 0; routes sourced by the local router: 32768.

NSF1 Awareness

Disabled2. If enabled, allows Layer 3 switches to continue forwarding packets from a neighboring NSF-capable router during hardware or software changes.

Route reflector

None configured.

Synchronization (BGP and IGP)

Disabled.

Table map update

Disabled.

Timers

Keepalive: 60 seconds; holdtime: 180 seconds.

1 Nonstop Forwarding
2 NSF Awareness can be enabled for IPv4 on switches with the license by enabling Graceful Restart.

Enabling BGP Routing

Before you begin


Note

To enable BGP, the switch or stack master must be running the IP services feature set.


Procedure

  Command or Action Purpose
Step 1

configure terminal

Example:


Device# configure terminal


Enters global configuration mode.

Step 2

ip routing

Example:


Device(config)# ip routing

Enables IP routing.

Step 3

router bgp autonomous-system

Example:


Device(config)# router bgp 45000

Enables a BGP routing process, assign it an AS number, and enter router configuration mode. The AS number can be from 1 to 65535, with 64512 to 65535 designated as private autonomous numbers.

Step 4

network network-number [mask network-mask] [route-map route-map-name]

Example:


Device(config)# network 10.108.0.0

Configures a network as local to this AS, and enter it in the BGP table.

Step 5

neighbor {ip-address | peer-group-name} remote-as number

Example:


Device(config)# neighbor 10.108.1.2 remote-as 65200

Adds an entry to the BGP neighbor table specifying that the neighbor identified by the IP address belongs to the specified AS.

For EBGP, neighbors are usually directly connected, and the IP address is the address of the interface at the other end of the connection.

For IBGP, the IP address can be the address of any of the router interfaces.

Step 6

neighbor {ip-address | peer-group-name} remove-private-as

Example:


Device(config)# neighbor 172.16.2.33 remove-private-as

(Optional) Removes private AS numbers from the AS-path in outbound routing updates.

Step 7

synchronization

Example:


Device(config)# synchronization

(Optional) Enables synchronization between BGP and an IGP.

Step 8

auto-summary

Example:


Device(config)# auto-summary

(Optional) Enables automatic network summarization. When a subnet is redistributed from an IGP into BGP, only the network route is inserted into the BGP table.

Step 9

bgp graceful-restart

Example:


Device(config)# bgp graceful-start

(Optional) Enables NSF awareness on switch. By default, NSF awareness is disabled.

Step 10

end

Example:


Device(config)# end


Returns to privileged EXEC mode.

Step 11

show ip bgp network network-number

Example:


Device# show ip bgp network 10.108.0.0

Verifies the configuration.

Step 12

show ip bgp neighbor

Example:


Device# show ip bgp neighbor

Verifies that NSF awareness (Graceful Restart) is enabled on the neighbor.

If NSF awareness is enabled on the switch and the neighbor, this message appears:

Graceful Restart Capability: advertised and received

If NSF awareness is enabled on the switch, but not on the neighbor, this message appears:

Graceful Restart Capability: advertised

Step 13

copy running-config startup-config

Example:


Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Managing Routing Policy Changes

To learn if a BGP peer supports the route refresh capability and to reset the BGP session:

Procedure

  Command or Action Purpose
Step 1

show ip bgp neighbors

Example:


Device# show ip bgp neighbors

Displays whether a neighbor supports the route refresh capability. When supported, this message appears for the router:

Received route refresh capability from peer.

Step 2

clear ip bgp {* | address | peer-group-name}

Example:


Device# clear ip bgp *

Resets the routing table on the specified connection.

  • Enter an asterisk (*) to specify that all connections be reset.

  • Enter an IP address to specify the connection to be reset.

  • Enter a peer group name to reset the peer group.

Step 3

clear ip bgp {* | address | peer-group-name} soft out

Example:


Device# clear ip bgp * soft out

(Optional) Performs an outbound soft reset to reset the inbound routing table on the specified connection. Use this command if route refresh is supported.

  • Enter an asterisk (*) to specify that all connections be reset.

  • Enter an IP address to specify the connection to be reset.

  • Enter a peer group name to reset the peer group.

Step 4

show ip bgp

Example:


Device# show ip bgp

Verifies the reset by checking information about the routing table and about BGP neighbors.

Step 5

show ip bgp neighbors

Example:


Device# show ip bgp neighbors

Verifies the reset by checking information about the routing table and about BGP neighbors.

Configuring BGP Decision Attributes

Procedure

  Command or Action Purpose
Step 1

configure terminal

Example:


Device# configure terminal


Enters global configuration mode.

Step 2

router bgp autonomous-system

Example:


Device(config)# router bgp 4500

Enables a BGP routing process, assign it an AS number, and enter router configuration mode.

Step 3

bgp best-path as-path ignore

Example:


Device(config-router)# bgp bestpath as-path ignore

(Optional) Configures the router to ignore AS path length in selecting a route.

Step 4

neighbor {ip-address | peer-group-name} next-hop-self

Example:


Device(config-router)# neighbor 10.108.1.1 next-hop-self

(Optional) Disables next-hop processing on BGP updates to a neighbor by entering a specific IP address to be used instead of the next-hop address.

Step 5

neighbor {ip-address | peer-group-name} weight weight

Example:


Device(config-router)# neighbor 172.16.12.1 weight 50

(Optional) Assign a weight to a neighbor connection. Acceptable values are from 0 to 65535; the largest weight is the preferred route. Routes learned through another BGP peer have a default weight of 0; routes sourced by the local router have a default weight of 32768.

Step 6

default-metric number

Example:


Device(config-router)# default-metric 300

(Optional) Sets a MED metric to set preferred paths to external neighbors. All routes without a MED will also be set to this value. The range is 1 to 4294967295. The lowest value is the most desirable.

Step 7

bgp bestpath med missing-as-worst

Example:


Device(config-router)# bgp bestpath med missing-as-worst

(Optional) Configures the switch to consider a missing MED as having a value of infinity, making the path without a MED value the least desirable path.

Step 8

bgp always-compare med

Example:


Device(config-router)# bgp always-compare-med

(Optional) Configures the switch to compare MEDs for paths from neighbors in different autonomous systems. By default, MED comparison is only done among paths in the same AS.

Step 9

bgp bestpath med confed

Example:


Device(config-router)# bgp bestpath med confed

(Optional) Configures the switch to consider the MED in choosing a path from among those advertised by different subautonomous systems within a confederation.

Step 10

bgp deterministic med

Example:


Device(config-router)# bgp deterministic med

(Optional) Configures the switch to consider the MED variable when choosing among routes advertised by different peers in the same AS.

Step 11

bgp default local-preference value

Example:


Device(config-router)# bgp default local-preference 200

(Optional) Change the default local preference value. The range is 0 to 4294967295; the default value is 100. The highest local preference value is preferred.

Step 12

maximum-paths number

Example:


Device(config-router)# maximum-paths 8

(Optional) Configures the number of paths to be added to the IP routing table. The default is to only enter the best path in the routing table. The range is from 1 to 16. Having multiple paths allows load-balancing among the paths. (Although the switch software allows a maximum of 32 equal-cost routes, the switch hardware will never use more than 16 paths per route.)

Step 13

end

Example:


Device(config)# end


Returns to privileged EXEC mode.

Step 14

show ip bgp

Example:


Device# show ip bgp

Verifies the reset by checking information about the routing table and about BGP neighbors.

Step 15

show ip bgp neighbors

Example:


Device# show ip bgp neighbors

Verifies the reset by checking information about the routing table and about BGP neighbors.

Step 16

copy running-config startup-config

Example:


Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Configuring BGP Filtering with Route Maps

Procedure

  Command or Action Purpose
Step 1

configure terminal

Example:


Device# configure terminal


Enters global configuration mode.

Step 2

route-map map-tag [permit | deny] [sequence-number]

Example:


Device(config)# route-map set-peer-address permit 10

Creates a route map, and enter route-map configuration mode.

Step 3

set ip next-hop ip-address [...ip-address] [peer-address]

Example:


Device(config)# set ip next-hop 10.1.1.3

(Optional) Sets a route map to disable next-hop processing

  • In an inbound route map, set the next hop of matching routes to be the neighbor peering address, overriding third-party next hops.

  • In an outbound route map of a BGP peer, set the next hop to the peering address of the local router, disabling the next-hop calculation.

Step 4

end

Example:


Device(config)# end


Returns to privileged EXEC mode.

Step 5

show route-map [map-name]

Example:


Device# show route-map

Displays all route maps configured or only the one specified to verify configuration.

Step 6

copy running-config startup-config

Example:


Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Configuring BGP Filtering by Neighbor

Procedure

  Command or Action Purpose
Step 1

configure terminal

Example:


Device# configure terminal


Enters global configuration mode.

Step 2

router bgp autonomous-system

Example:


Device(config)# router bgp 109

Enables a BGP routing process, assign it an AS number, and enter router configuration mode.

Step 3

neighbor {ip-address | peer-group name} distribute-list { access-list-number | name} {in | out}

Example:


Device(config-router)# neighbor 172.16.4.1 distribute-list 39 in

(Optional) Filters BGP routing updates to or from neighbors as specified in an access list.

Note 

You can also use the neighbor prefix-list router configuration command to filter updates, but you cannot use both commands to configure the same BGP peer.

Step 4

neighbor {ip-address | peer-group name} route-map map-tag {in | out}

Example:


Device(config-router)# neighbor 172.16.70.24 route-map internal-map in

(Optional) Applies a route map to filter an incoming or outgoing route.

Step 5

end

Example:


Device(config)# end


Returns to privileged EXEC mode.

Step 6

show ip bgp neighbors

Example:


Device# show ip bgp neighbors

Verifies the configuration.

Step 7

copy running-config startup-config

Example:


Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Configuring BGP Filtering by Access Lists and Neighbors

Another method of filtering is to specify an access list filter on both incoming and outbound updates, based on the BGP autonomous system paths. Each filter is an access list based on regular expressions. (See the “Regular Expressions” appendix in the Cisco IOS Dial Technologies Command Reference, Release 12.4 for more information on forming regular expressions.) To use this method, define an autonomous system path access list, and apply it to updates to and from particular neighbors.

Procedure

  Command or Action Purpose
Step 1

configure terminal

Example:


Device# configure terminal


Enters global configuration mode.

Step 2

ip as-path access-list access-list-number {permit | deny} as-regular-expressions

Example:


Device(config)# ip as-path access-list 1 deny _65535_

Defines a BGP-related access list.

Step 3

router bgp autonomous-system

Example:


Device(config)# router bgp 110

Enters BGP router configuration mode.

Step 4

neighbor {ip-address | peer-group name} filter-list {access-list-number | name} {in | out | weight weight}

Example:


Device(config-router)# neighbor 172.16.1.1 filter-list 1 out

Establishes a BGP filter based on an access list.

Step 5

end

Example:


Device(config)# end


Returns to privileged EXEC mode.

Step 6

show ip bgp neighbors [paths regular-expression]

Example:


Device# show ip bgp neighbors

Verifies the configuration.

Step 7

copy running-config startup-config

Example:


Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Configuring Prefix Lists for BGP Filtering

You do not need to specify a sequence number when removing a configuration entry. Show commands include the sequence numbers in their output.

Before using a prefix list in a command, you must set up the prefix list.

Procedure

  Command or Action Purpose
Step 1

configure terminal

Example:


Device# configure terminal


Enters global configuration mode.

Step 2

ip prefix-list list-name [seq seq-value] deny | permit network/len [ge ge-value] [le le-value]

Example:


Device(config)# ip prefix-list BLUE permit 172.16.1.0/24

Creates a prefix list with an optional sequence number to deny or permit access for matching conditions. You must enter at least one permit or deny clause.

  • network/len is the network number and length (in bits) of the network mask.

  • (Optional) ge and le values specify the range of the prefix length to be matched.The specified ge-value and le-value must satisfy this condition: len < ge-value < le-value < 32

Step 3

ip prefix-list list-name seq seq-value deny | permit network/len [ge ge-value] [le le-value]

Example:


Device(config)# ip prefix-list BLUE seq 10 permit 172.24.1.0/24

(Optional) Adds an entry to a prefix list, and assign a sequence number to the entry.

Step 4

end

Example:


Device(config)# end


Returns to privileged EXEC mode.

Step 5

show ip prefix list [detail | summary] name [network/len] [seq seq-num] [longer] [first-match]

Example:


Device# show ip prefix list summary test

Verifies the configuration by displaying information about a prefix list or prefix list entries.

Step 6

copy running-config startup-config

Example:


Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Configuring BGP Community Filtering

By default, no COMMUNITIES attribute is sent to a neighbor. You can specify that the COMMUNITIES attribute be sent to the neighbor at an IP address by using the neighbor send-community router configuration command.

Procedure

  Command or Action Purpose
Step 1

configure terminal

Example:


Device# configure terminal


Enters global configuration mode.

Step 2

ip community-list community-list-number {permit | deny} community-number

Example:


Device(config)# ip community-list 1 permit 50000:10

Creates a community list, and assigns it a number.

  • The community-list-number is an integer from 1 to 99 that identifies one or more permit or deny groups of communities.

  • The community-number is the number configured by a set community route-map configuration command.

Step 3

router bgp autonomous-system

Example:


Device(config)# router bgp 108

Enters BGP router configuration mode.

Step 4

neighbor {ip-address | peer-group name} send-community

Example:


Device(config-router)# neighbor 172.16.70.23 send-community

Specifies that the COMMUNITIES attribute be sent to the neighbor at this IP address.

Step 5

set comm-list list-num delete

Example:


Device(config-router)# set comm-list 500 delete

(Optional) Removes communities from the community attribute of an inbound or outbound update that match a standard or extended community list specified by a route map.

Step 6

exit

Example:


Device(config-router)# end

Returns to global configuration mode.

Step 7

ip bgp-community new-format

Example:


Device(config)# ip bgp-community new format

(Optional) Displays and parses BGP communities in the format AA:NN.

A BGP community is displayed in a two-part format 2 bytes long. The Cisco default community format is in the format NNAA. In the most recent RFC for BGP, a community takes the form AA:NN, where the first part is the AS number and the second part is a 2-byte number.

Step 8

end

Example:


Device(config)# end


Returns to privileged EXEC mode.

Step 9

show ip bgp community

Example:


Device# show ip bgp community

Verifies the configuration.

Step 10

copy running-config startup-config

Example:


Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Configuring BGP Neighbors and Peer Groups

To assign configuration options to an individual neighbor, specify any of these router configuration commands by using the neighbor IP address. To assign the options to a peer group, specify any of the commands by using the peer group name. You can disable a BGP peer or peer group without removing all the configuration information by using the neighbor shutdown router configuration command.

Procedure

  Command or Action Purpose
Step 1

configure terminal

Example:


Device# configure terminal


Enters global configuration mode.

Step 2

router bgp autonomous-system

Enters BGP router configuration mode.

Step 3

neighbor peer-group-name peer-group

Creates a BGP peer group.

Step 4

neighbor ip-address peer-group peer-group-name

Makes a BGP neighbor a member of the peer group.

Step 5

neighbor {ip-address | peer-group-name} remote-as number

Specifies a BGP neighbor. If a peer group is not configured with a remote-as number , use this command to create peer groups containing EBGP neighbors. The range is 1 to 65535.

Step 6

neighbor {ip-address | peer-group-name} description text

(Optional) Associates a description with a neighbor.

Step 7

neighbor {ip-address | peer-group-name} default-originate [route-map map-name]

(Optional) Allows a BGP speaker (the local router) to send the default route 0.0.0.0 to a neighbor for use as a default route.

Step 8

neighbor {ip-address | peer-group-name} send-community

(Optional) Specifies that the COMMUNITIES attribute be sent to the neighbor at this IP address.

Step 9

neighbor {ip-address | peer-group-name} update-source interface

(Optional) Allows internal BGP sessions to use any operational interface for TCP connections.

Step 10

neighbor {ip-address | peer-group-name} ebgp-multihop

(Optional) Allows BGP sessions, even when the neighbor is not on a directly connected segment. The multihop session is not established if the only route to the multihop peer’s address is the default route (0.0.0.0).

Step 11

neighbor {ip-address | peer-group-name} local-as number

(Optional) Specifies an AS number to use as the local AS. The range is 1 to 65535.

Step 12

neighbor {ip-address | peer-group-name} advertisement-interval seconds

(Optional) Sets the minimum interval between sending BGP routing updates.

Step 13

neighbor {ip-address | peer-group-name} maximum-prefix maximum [threshold]

(Optional) Controls how many prefixes can be received from a neighbor. The range is 1 to 4294967295. The threshold (optional) is the percentage of maximum at which a warning message is generated. The default is 75 percent.

Step 14

neighbor {ip-address | peer-group-name} next-hop-self

(Optional) Disables next-hop processing on the BGP updates to a neighbor.

Step 15

neighbor {ip-address | peer-group-name} password string

(Optional) Sets MD5 authentication on a TCP connection to a BGP peer. The same password must be configured on both BGP peers, or the connection between them is not made.

Step 16

neighbor {ip-address | peer-group-name} route-map map-name {in | out}

(Optional) Applies a route map to incoming or outgoing routes.

Step 17

neighbor {ip-address | peer-group-name} send-community

(Optional) Specifies that the COMMUNITIES attribute be sent to the neighbor at this IP address.

Step 18

neighbor {ip-address | peer-group-name} timers keepalive holdtime

(Optional) Sets timers for the neighbor or peer group.

  • The keepalive interval is the time within which keepalive messages are sent to peers. The range is 1 to 4294967295 seconds; the default is 60.

  • The holdtime is the interval after which a peer is declared inactive after not receiving a keepalive message from it. The range is 1 to 4294967295 seconds; the default is 180.

Step 19

neighbor {ip-address | peer-group-name} weight weight

(Optional) Specifies a weight for all routes from a neighbor.

Step 20

neighbor {ip-address | peer-group-name} distribute-list {access-list-number | name} {in | out}

(Optional) Filter BGP routing updates to or from neighbors, as specified in an access list.

Step 21

neighbor {ip-address | peer-group-name} filter-list access-list-number {in | out | weight weight}

(Optional) Establish a BGP filter.

Step 22

neighbor {ip-address | peer-group-name} version value

(Optional) Specifies the BGP version to use when communicating with a neighbor.

Step 23

neighbor {ip-address | peer-group-name} soft-reconfiguration inbound

(Optional) Configures the software to start storing received updates.

Step 24

end

Example:


Device(config)# end


Returns to privileged EXEC mode.

Step 25

show ip bgp neighbors

Verifies the configuration.

Step 26

copy running-config startup-config

Example:


Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Configuring Aggregate Addresses in a Routing Table

Procedure

  Command or Action Purpose
Step 1

configure terminal

Example:


Device# configure terminal


Enters global configuration mode.

Step 2

router bgp autonomous-system

Example:


Device(config)# router bgp 106

Enters BGP router configuration mode.

Step 3

aggregate-address address mask

Example:


Device(config-router)# aggregate-address 10.0.0.0 255.0.0.0

Creates an aggregate entry in the BGP routing table. The aggregate route is advertised as coming from the AS, and the atomic aggregate attribute is set to indicate that information might be missing.

Step 4

aggregate-address address mask as-set

Example:


Device(config-router)# aggregate-address 10.0.0.0 255.0.0.0 as-set

(Optional) Generates AS set path information. This command creates an aggregate entry following the same rules as the previous command, but the advertised path will be an AS_SET consisting of all elements contained in all paths. Do not use this keyword when aggregating many paths because this route must be continually withdrawn and updated.

Step 5

aggregate-address address-mask summary-only

Example:


Device(config-router)# aggregate-address 10.0.0.0 255.0.0.0 summary-only

(Optional) Advertises summary addresses only.

Step 6

aggregate-address address mask suppress-map map-name

Example:


Device(config-router)# aggregate-address 10.0.0.0 255.0.0.0 suppress-map map1

(Optional) Suppresses selected, more specific routes.

Step 7

aggregate-address address mask advertise-map map-name

Example:


Device(config-router)# aggregate-address 10.0.0.0 255.0.0.0 advertise-map map2

(Optional) Generates an aggregate based on conditions specified by the route map.

Step 8

aggregate-address address mask attribute-map map-name

Example:


Device(config-router)# aggregate-address 10.0.0.0 255.0.0.0 attribute-map map3

(Optional) Generates an aggregate with attributes specified in the route map.

Step 9

end

Example:


Device(config)# end


Returns to privileged EXEC mode.

Step 10

show ip bgp neighbors [advertised-routes]

Example:


Device# show ip bgp neighbors

Verifies the configuration.

Step 11

copy running-config startup-config

Example:


Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Configuring Routing Domain Confederations

You must specify a confederation identifier that acts as the autonomous system number for the group of autonomous systems.

Procedure

  Command or Action Purpose
Step 1

configure terminal

Example:


Device# configure terminal


Enters global configuration mode.

Step 2

router bgp autonomous-system

Example:


Device(config)# router bgp 100

Enters BGP router configuration mode.

Step 3

bgp confederation identifier autonomous-system

Example:


Device(config)# bgp confederation identifier 50007

Configures a BGP confederation identifier.

Step 4

bgp confederation peers autonomous-system [autonomous-system ...]

Example:


Device(config)# bgp confederation peers 51000 51001 51002

Specifies the autonomous systems that belong to the confederation and that will be treated as special EBGP peers.

Step 5

end

Example:


Device(config)# end


Returns to privileged EXEC mode.

Step 6

show ip bgp neighbor

Example:


Device# show ip bgp neighbor

Verifies the configuration.

Step 7

show ip bgp network

Example:


Device# show ip bgp network

Verifies the configuration.

Step 8

copy running-config startup-config

Example:


Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Configuring BGP Route Reflectors

Procedure

  Command or Action Purpose
Step 1

configure terminal

Example:


Device# configure terminal


Enters global configuration mode.

Step 2

router bgp autonomous-system

Example:


Device(config)# router bgp 101

Enters BGP router configuration mode.

Step 3

neighbor {ip-address | peer-group-name} route-reflector-client

Example:


Device(config-router)# neighbor 172.16.70.24 route-reflector-client

Configures the local router as a BGP route reflector and the specified neighbor as a client.

Step 4

bgp cluster-id cluster-id

Example:


Device(config-router)# bgp cluster-id 10.0.1.2

(Optional) Configures the cluster ID if the cluster has more than one route reflector.

Step 5

no bgp client-to-client reflection

Example:


Device(config-router)# no bgp client-to-client reflection

(Optional) Disables client-to-client route reflection. By default, the routes from a route reflector client are reflected to other clients. However, if the clients are fully meshed, the route reflector does not need to reflect routes to clients.

Step 6

end

Example:


Device(config)# end


Returns to privileged EXEC mode.

Step 7

show ip bgp

Example:


Device# show ip bgp

Verifies the configuration. Displays the originator ID and the cluster-list attributes.

Step 8

copy running-config startup-config

Example:


Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Configuring Route Dampening

Procedure

  Command or Action Purpose
Step 1

configure terminal

Example:


Device# configure terminal


Enters global configuration mode.

Step 2

router bgp autonomous-system

Example:


Device(config)# router bgp 100

Enters BGP router configuration mode.

Step 3

bgp dampening

Example:


Device(config-router)# bgp dampening

Enables BGP route dampening.

Step 4

bgp dampening half-life reuse suppress max-suppress [route-map map]

Example:


Device(config-router)# bgp dampening 30 1500 10000 120

(Optional) Changes the default values of route dampening factors.

Step 5

end

Example:


Device(config)# end


Returns to privileged EXEC mode.

Step 6

show ip bgp flap-statistics [{regexp regexp} | {filter-list list} | {address mask [longer-prefix]}]

Example:


Device# show ip bgp flap-statistics

(Optional) Monitors the flaps of all paths that are flapping. The statistics are deleted when the route is not suppressed and is stable.

Step 7

show ip bgp dampened-paths

Example:


Device# show pi bgp dampened-paths

(Optional) Displays the dampened routes, including the time remaining before they are suppressed.

Step 8

clear ip bgp flap-statistics [{regexp regexp} | {filter-list list} | {address mask [longer-prefix]}

Example:


Device# clear ip bgp flap-statistics

(Optional) Clears BGP flap statistics to make it less likely that a route will be dampened.

Step 9

clear ip bgp dampening

Example:


Device# clear ip bgp dampening

(Optional) Clears route dampening information, and unsuppress the suppressed routes.

Step 10

copy running-config startup-config

Example:


Device# copy running-config startup-config 


(Optional) Saves your entries in the configuration file.

Monitoring and Maintaining BGP

You can remove all contents of a particular cache, table, or database. This might be necessary when the contents of the particular structure have become or are suspected to be invalid.

You can display specific statistics, such as the contents of BGP routing tables, caches, and databases. You can use the information to get resource utilization and solve network problems. You can also display information about node reachability and discover the routing path your device’s packets are taking through the network.

The table given below lists the privileged EXEC commands for clearing and displaying BGP. For explanations of the display fields, see the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.4.

Table 11. IP BGP Clear and Show Commands

clear ip bgp address

Resets a particular BGP connection.

clear ip bgp *

Resets all BGP connections.

clear ip bgp peer-group tag

Removes all members of a BGP peer group.

show ip bgp prefix

Displays peer groups and peers not in peer groups to which the prefix has been advertised. Also displays prefix attributes such as the next hop and the local prefix.

show ip bgp cidr-only

Displays all BGP routes that contain subnet and supernet network masks.

show ip bgp community [community-number] [exact]

Displays routes that belong to the specified communities.

show ip bgp community-list community-list-number [exact-match]

Displays routes that are permitted by the community list.

show ip bgp filter-list access-list-number

Displays routes that are matched by the specified AS path access list.

show ip bgp inconsistent-as

Displays the routes with inconsistent originating autonomous systems.

show ip bgp regexp regular-expression

Displays the routes that have an AS path that matches the specified regular expression entered on the command line.

show ip bgp

Displays the contents of the BGP routing table.

show ip bgp neighbors [address]

Displays detailed information on the BGP and TCP connections to individual neighbors.

show ip bgp neighbors [address] [advertised-routes | dampened-routes | flap-statistics | paths regular-expression | received-routes | routes]

Displays routes learned from a particular BGP neighbor.

show ip bgp paths

Displays all BGP paths in the database.

show ip bgp peer-group [tag] [summary]

Displays information about BGP peer groups.

show ip bgp summary

Displays the status of all BGP connections.

The bgp log-neighbor changes command is enabled by default. It allows to log messages that are generated when a BGP neighbor resets, comes up, or goes down.

Configuration Examples for BGP

Example: Configuring BGP on Routers

These examples show how to configure BGP on the routers in the figure below, Figure 6. EBGP, IBGP, and Multiple Autonomous Systems

Router A:


Device(config)# router bgp 100
Device(config-router)# neighbor 129.213.1.1 remote-as 200

Router B:


Device(config)# router bgp 200
Device(config-router)# neighbor 129.213.1.2 remote-as 100
Device(config-router)# neighbor 175.220.1.2 remote-as 200

Router C:


Device(config)# router bgp 200
Device(config-router)# neighbor 175.220.212.1 remote-as 200
Device(config-router)# neighbor 192.208.10.1 remote-as 300

Router D:


Device(config)# router bgp 300
Device(config-router)# neighbor 192.208.10.2 remote-as 200

To verify that BGP peers are running, use the show ip bgp neighbors privileged EXEC command. This is the output of this command on Router A:


Device# show ip bgp neighbors

BGP neighbor is 129.213.1.1, remote AS 200, external link
 BGP version 4, remote router ID 175.220.212.1
 BGP state = established, table version = 3, up for 0:10:59
 Last read 0:00:29, hold time is 180, keepalive interval is 60 seconds
 Minimum time between advertisement runs is 30 seconds
 Received 2828 messages, 0 notifications, 0 in queue
 Sent 2826 messages, 0 notifications, 0 in queue
 Connections established 11; dropped 10

Anything other than state = established means that the peers are not running. The remote router ID is the highest IP address on that router (or the highest loopback interface). Each time the table is updated with new information, the table version number increments. A table version number that continually increments means that a route is flapping, causing continual routing updates.

For exterior protocols, a reference to an IP network from the network router configuration command controls only which networks are advertised. This is in contrast to Interior Gateway Protocols (IGPs), such as EIGRP, which also use the network command to specify where to send updates.

For detailed descriptions of BGP configuration, see the “IP Routing Protocols” part of the Cisco IOS IP Configuration Guide, Release 12.4. For details about specific commands, see the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.4.

Information About ISO CLNS Routing

Connectionless Routing

The International Organization for Standardization (ISO) Connectionless Network Service (CLNS) protocol is a standard for the network layer of the Open System Interconnection (OSI) model. Addresses in the ISO network architecture are referred to as network service access point (NSAP) addresses and network entity titles (NETs). Each node in an OSI network has one or more NETs. In addition, each node has many NSAP addresses.

When you enable connectionless routing on the Device by using the clns routing global configuration command, the Device makes only forwarding decisions, with no routing-related functionality. For dynamic routing, you must also enable a routing protocol. The Device supports the Intermediate System-to-Intermediate System (IS-IS) dynamic routing protocol that is based on the OSI routing protocol for ISO CLNS networks.

When dynamically routing, you use IS-IS. This routing protocol supports the concept of areas. Within an area, all routers know how to reach all the system IDs. Between areas, routers know how to reach the proper area. IS-IS supports two levels of routing: station routing (within an area) and area routing (between areas).

The key difference between the ISO IGRP and IS-IS NSAP addressing schemes is in the definition of area addresses. Both use the system ID for Level 1 routing (routing within an area). However, they differ in the way addresses are specified for area routing. An ISO IGRP NSAP address includes three separate fields for routing: the domain, area, and system ID. An IS-IS address includes two fields: a single continuous area field (comprising the domain and area fields) and the system ID.


Note

For more detailed information about ISO CLNS, see the Cisco IOS Apollo Domain, Banyan VINES, DECnet, ISO CLNS and XNS Configuration Guide, Release 12.4. For complete syntax and usage information for the commands used in this chapter, see the Cisco IOS Apollo Domain, Banyan VINES, DECnet, ISO CLNS and XNS Command Reference, Release 12.4, use the IOS command reference master index, or search online.


IS-IS Dynamic Routing

IS-IS is an ISO dynamic routing protocol (described in ISO 105890). Unlike other routing protocols, enabling IS-IS requires that you create an IS-IS routing process and assign it to a specific interface, rather than to a network. You can specify more than one IS-IS routing process per Layer 3 Device or router by using the multiarea IS-IS configuration syntax. You then configure the parameters for each instance of the IS-IS routing process.

Small IS-IS networks are built as a single area that includes all the routers in the network. As the network grows larger, it is usually reorganized into a backbone area made up of the connected set of all Level 2 routers from all areas, which is in turn connected to local areas. Within a local area, routers know how to reach all system IDs. Between areas, routers know how to reach the backbone, and the backbone routers know how to reach other areas.

Routers establish Level 1 adjacencies to perform routing within a local area (station routing). Routers establish Level 2 adjacencies to perform routing between Level 1 areas (area routing).

A single Cisco router can participate in routing in up to 29 areas and can perform Level 2 routing in the backbone. In general, each routing process corresponds to an area. By default, the first instance of the routing process configured performs both Level 1and Level 2 routing. You can configure additional router instances, which are automatically treated as Level 1 areas. You must configure the parameters for each instance of the IS-IS routing process individually.

For IS-IS multiarea routing, you can configure only one process to perform Level 2 routing, although you can define up to 29 Level 1 areas for each Cisco unit. If Level 2 routing is configured on any process, all additional processes are automatically configured as Level 1. You can configure this process to perform Level 1 routing at the same time. If Level 2 routing is not desired for a router instance, remove the Level 2 capability using the is-type global configuration command. Use the is-type command also to configure a different router instance as a Level 2 router.


Note

For more detailed information about IS-IS, see the “IP Routing Protocols” chapter of the Cisco IOS IP Configuration Guide, Release 12.4. For complete syntax and usage information for the commands used in this section, see the Cisco IOS IP Command Reference, Release 12.4.


Nonstop Forwarding Awareness

The integrated IS-IS Nonstop Forwarding (NSF) Awareness feature is supported for IPv4G. The feature allows customer premises equipment (CPE) devices that are NSF-aware to help NSF-capable devices perform nonstop forwarding of packets. The local device is not necessarily performing NSF, but its NSF awareness capability allows the integrity and accuracy of the routing database and the link-state database on the neighboring NSF-capable device to be maintained during the switchover process.

The integrated IS-IS Nonstop Forwarding (NSF) Awareness feature is automatically enabled and requires no configuration.

IS-IS Global Parameters

The following are the optional IS-IS global parameters that you can configure:

  • You can force a default route into an IS-IS routing domain by configuring a default route that is controlled by a route map. You can also specify the other filtering options that are configurable under a route map.

  • You can configure the device to ignore IS-IS link-state packets (LSPs) that are received with internal checksum errors, or to purge corrupted LSPs, and cause the initiator of the LSP to regenerate it.

  • You can assign passwords to areas and domains.

  • You can create aggregate addresses that are represented in the routing table by a summary address (based on route summarization). Routes learned from other routing protocols can also be summarized. The metric used to advertise the summary is the smallest metric of all the specific routes.

  • You can set an overload bit.

  • You can configure the LSP refresh interval and the maximum time that an LSP can remain in the device database without a refresh.

  • You can set the throttling timers for LSP generation, shortest path first computation, and partial route computation.

  • You can configure the device to generate a log message when an IS-IS adjacency changes state (Up or Down).

  • If a link in the network has a maximum transmission unit (MTU) size of less than 1500 bytes, you can lower the LSP MTU so that routing still occurs.

  • You can use the partition avoidance device configuration command to prevent an area from becoming partitioned when full connectivity is lost among a Level 1-2 border device, adjacent Level 1 devices, and end hosts.

IS-IS Interface Parameters

You can optionally configure certain interface-specific IS-IS parameters independently from other attached devices. However, if you change default value, such as multipliers and time intervals, it makes sense to also change them on multiple devices and interfaces. Most of the interface parameters can be configured for level 1, level 2, or both.

The following are the interface-level parameters that you can configure:

  • The default metric on the interface that is used as a value for the IS-IS metric and assigned when quality of service (QoS) routing is not performed.

  • The hello interval (length of time between hello packets sent on the interface) or the default hello packet multiplier used on the interface to determine the hold time sent in IS-IS hello packets. The hold time determines how long a neighbor waits for another hello packet before declaring the neighbor down. This determines how quickly a failed link or neighbor is detected so that routes can be recalculated. Change the hello multiplier in circumstances where hello packets are lost frequently and IS-IS adjacencies are failing unnecessarily. You can raise the hello multiplier and lower the hello interval correspondingly to make the hello protocol more reliable, without increasing the time required to detect a link failure.

  • Other time intervals:

    • Complete sequence number PDU (CSNP) interval—CSNPs are sent by the designated device to maintain database synchronization.

    • Retransmission interval—This is the time between retransmission of IS-IS LSPs for point-to-point links.

    • IS-IS LSP retransmission throttle interval—This is the maximum rate (number of milliseconds between packets) at which IS-IS LSPs are resent on point-to-point links. This interval is different from the retransmission interval, which is the time between successive retransmissions of the same LSP.

  • Designated device-election priority, which allows you to reduce the number of adjacencies required on a multiaccess network, which in turn reduces the amount of routing protocol traffic and the size of the topology database.

  • The interface circuit type, which is the type of adjacency required for neighbors on the specified interface.

  • Password authentication for the interface.

How to Configure ISO CLNS Routing

Default IS-IS Configuration

Table 12. Default IS-IS Configuration

Feature

Default Setting

Ignore link-state PDU (LSP) errors

Enabled.

IS-IS type

Conventional IS-IS—The router acts as both a Level 1 (station) and a Level 2 (area) router.

Multiarea IS-IS—The first instance of the IS-IS routing process is a Level 1-2 router. Remaining instances are Level 1 routers.

Default-information originate

Disabled.

Log IS-IS adjacency state changes.

Disabled.

LSP generation throttling timers

Maximum interval between two consecutive occurrences—5000 milliseconds.

Initial LSP generation delay—50 milliseconds.

Hold time between the first and second LSP generation—200 milliseconds.

LSP maximum lifetime (without a refresh)

1200 seconds (20 minutes) before the LSP packet is deleted.

LSP refresh interval

Every 900 seconds (15 minutes).

Maximum LSP packet size

1497 bytes.

NSF Awareness

Enabled. Allows Layer 3 devicess to continue forwarding packets from a neighboring Nonstop Forwarding-capable router during hardware or software changes.

Partial route computation (PRC) throttling timers

Maximum PRC wait interval—5000 milliseconds.

Initial PRC calculation delay after a topology change—50 milliseconds.

Hold time between the first and second PRC calculation—200 milliseconds.

Partition avoidance

Disabled.

Password

No area or domain password is defined, and authentication is disabled.

Set-overload-bit

Disabled. When enabled, if no arguments are entered, the overload bit is set immediately and remains set until you enter the no set-overload-bit command.

Shortest path first (SPF) throttling timers

Maximum interval between consecutive SFPs—5000 milliseconds.

Initial SFP calculation after a topology change—200 milliseconds.

Hold time between the first and second SFP calculation—50 milliseconds.

Summary-address

Disabled.

Enabling IS-IS Routing

To enable IS-IS, you specify a name and NET for each routing process. You then enable IS-IS routing on the interface and specify the area for each instance of the routing process.

Procedure

  Command or Action Purpose
Step 1

configure terminal

Example:


Device# configure terminal


Enters global configuration mode.

Step 2

clns routing

Example:


Device(config)# clns routing

Enables ISO connectionless routing on the switch.

Step 3

router isis [area tag]

Example:


Device(config)# router isis tag1

Enables the IS-IS routing for the specified routing process and enter IS-IS routing configuration mode.

(Optional) Use the area tag argument to identify the area to which the IS-IS router is assigned. You must enter a value if you are configuring multiple IS-IS areas.

The first IS-IS instance configured is Level 1-2 by default. Later instances are automatically Level 1. You can change the level of routing by using the is-type global configuration command.

Step 4

net network-entity-title

Example:


Device(config-router)# net 47.0004.004d.0001.0001.0c11.1111.00

Configures the NETs for the routing process. If you are configuring multiarea IS-IS, specify a NET for each routing process. You can specify a name for a NET and for an address.

Step 5

is-type {level-1 | level-1-2 | level-2-only}

Example:


Device(config-router)# is-type level-2-only

(Optional) Configures the router to act as a Level 1 (station) router, a Level 2 (area) router for multi-area routing, or both (the default):

  • level-1 —act as a station router only

  • level-1-2 —act as both a station router and an area router

  • level 2 —act as an area router only

Step 6

exit

Example:


Device(config-router)# end

Returns to global configuration mode.

Step 7

interface interface-id

Example:


Device(config)# interface gigabitethernet 1/0/1

Specifies an interface to route IS-IS, and enter interface configuration mode. If the interface is not already configured as a Layer 3 interface, enter the no switchport command to put it into Layer 3 mode.

Step 8

ip router isis [area tag]

Example:


Device(config-if)# ip router isis tag1

Configures an IS-IS routing process for ISO CLNS on the interface and attach an area designator to the routing process.

Step 9

clns router isis [area tag]

Example:


Device(config-if)# clns router isis tag1

Enables ISO CLNS on the interface.

Step 10

ip address ip-address-mask

Example:


Device(config-if)# ip address 10.0.0.5 255.255.255.0

Define the IP a