Published On: August 6ᵗʰ, 2019 02:03

IP Addressing: NAT Configuration Guide, Cisco IOS XE Release 3S

Carrier Grade Network Address Translation (CGN) is a large-scale NAT that translates private IPv4 addresses into public IPv4 addresses. CGN employs Network Address and Port Translation methods to aggregate multiple private IPv4 addresses into fewer public IPv4 addresses.

This module provides an overview of CGN and describes how to configure CGN.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/​go/​cfn. An account on Cisco.com is not required.

Restrictions for Carrier Grade Network Address Translation

  • Asymmetric routing with box-to-box (B2B) redundancy is not supported in Carrier Grade Network Address Translation (CGN) mode.

  • B2B redundancy is not supported on broadband with CGN; B2B is supported on standalone CGN.

  • Broadband is not supported with traditional NAT.

  • CGN does not support IP sessions.

  • NAT outside mappings are disabled automatically when CGN operating mode is configured using the ip nat settings mode cgn command.

  • CGN does not support integration with Cisco Performance Routing (PfR). Commands with the oer keyword are not supported. For example, the ip nat inside source route-map pool overload oer and the ip nat inside source list pool overload oer commands are not supported.

  • The match-in-vrf keyword for intra-VPN NAT is not supported with CGN.

Information About Carrier Grade Network Address Translation

Carrier Grade NAT Overview

Network Address Translation (NAT) is positioned between a private and public IP network and uses nonglobal, private IP addresses and a public IP address for translation. NAT dynamically maps one or more private IP addresses into one or more public (globally routable) IP addresses that use Network Address and Port Translation (NAPT) techniques. Traditionally, NAT boxes are deployed in residential home gateways (HGWs) to translate multiple private IP addresses that are configured on multiple devices inside the home to a single public IP address that is configured and provisioned on the HGW by the service provider. Service providers deploy NAT in such a way that multiple subscribers can share a single global IP address. The service provider NAT scales to several millions of NAT translations, making it a Carrier Grade NAT (CGN).

In CGN, packets that traverse from inside the network to outside require only the source address port translation; destination address port translation is not required. CGN can be standalone like traditional NAT or you can use it along with broadband access aggregation. CGN coexits with Intelligent Services Gateway (ISG) features such as Layer 4 Redirect and subscriber services such as traffic classes.

You can configure CGN by using the ip nat settings mode cgn command. Use the ip nat settings mode default command to change to the default or traditional NAT operating mode. In the CGN mode, you cannot configure any NAT outside mappings. However, when you change from the default NAT mode to CGN mode, all existing outside mappings have to be removed. Use the no ip nat settings support mapping outside command to remove all outside mappings and to prevent any new outside mappings from being configured. You can also remove outside mappings by using the no form of commands used to configure NAT outside.

CGN increases the scalability of the number of NAT translations that can be supported because destination information is not stored.

CGN supports the following:

  • All application-level gateways (ALGs) that are supported by traditional NAT. For more information about supported ALGs, see the Using Application-Level Gateways with NAT module of the IP Addressing: NAT Configuration Guide.

  • Endpoint independent mapping and endpoint independent filtering.

  • Hairpinning by using VRF-Aware Software Infrastructure (VASI) and policy-based routing (PBR). Hairpinning occurs when two subscribers are behind the same NAT device but can see each other only by using the global IP address.

  • Interbox and intrabox redundancy.

  • Lawful intercept.

  • Logging of NAT high-speed logging (HSL) records. For more information about HSL, see the section “High-Speed Logging for NAT” in the Maintaining and Monitoring NAT module of the IP Addressing: NAT Configuration Guide.

  • Multihoming, which is the ability to support multiple outside interfaces to provide connectivity through redundant or standby exit points. Depending on the configured routing topology, any exit interface that is marked as an outside interface can use a translation that was created previously.

  • TCP timeout value of 2 hours and 4 minutes.

  • VPN routing and forwarding (VRF)-aware NAT.

Carrier Grade NAT Support for Broadband Access Aggregation

You can configure Carrier Grade Network Address Translation (CGN) as an independent feature or use CGN along with broadband access aggregation.

Broadband access aggregation enables connections between multiple technologies such as cable, digital subscriber line (DSL), Ethernet, ISDN, and wireless devices that are connected to corporate VPNs, third-party applications, and the Internet.

PPP over Ethernet (PPPoE) connects hosts on a network over a simple bridging device to a remote aggregation concentrator. PPPoE is the predominant access protocol in broadband networks worldwide.

For PPPoE to work with CGN, virtual templates and the RADIUS server must support the Network Address Translation (NAT) inside configuration. The NAT inside configuration must be downloaded as part of the RADIUS authentication. Configure the ip nat inside command on the virtual template, which gets cloned into a virtual access interface that inherits the ip nat inside configuration. For the RADIUS server to support the NAT inside configuration, configure the aaa policy interface-config allow-subinterface command or configure the Cisco attribute-value pairs (AV pairs) “lcp:allow-subinterface=yes” and “lcp:interface-config=ip nat inside” in the RADIUS profile on a per-subscriber basis.

You can terminate a PPPoE session either in the global routing table or at a VRF instance.

CGN supports dual-stack (IPv4 and IPv6) PPP sessions. However, only IPv4 traffic is subject to NAT. The IPv6 traffic is not translated; it is routed as per the IPv6 routing configuration.

How to Configure Carrier Grade Network Address Translation

Based on your network configuration, you can configure static, dynamic, or dynamic PAT Carrier Grade NAT.


Note


You must use at least one of the configurations described in the following tasks for Carrier Grade NAT to work.


Configuring Static Carrier Grade NAT

Static address translation (static NAT) allows one-to-one mapping between local and global addresses. Use the ip nat inside source static command to enable static NAT of the inside source address.

SUMMARY STEPS

1. enable

2. configure terminal

3. ip nat settings mode cgn

4. ip nat inside source static local-ip global-ip

5. interface gigabitethernet card/spaslot/port.subinterface-number

6. ip nat inside

7. exit

8. interface type number

9. ip nat outside

10. end

11. show ip nat translations [verbose]


DETAILED STEPS
  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 nat settings mode cgn


Example:
Device(config)# ip nat settings mode cgn
 

Enables CGN operating mode.

 
Step 4 ip nat inside source static local-ip global-ip


Example:
Device(config)# ip nat inside source static 192.168.2.1 192.168.34.2
 

Enables static Carrier Grade NAT of the inside source address.

 
Step 5 interface gigabitethernet card/spaslot/port.subinterface-number


Example:
Device(config)# interface gigabitethernet 0/0/4
 

Configures an interface and enters interface configuration mode.

Note   

The NAT inside network can be applied to interface virtual-template when the router is used for broadband aggregation deployment.

 
Step 6 ip nat inside


Example:
Device(config-if)# ip nat inside
 

Indicates that the interface is connected to the inside network (the network that is subject to NAT translation).

 
Step 7 exit


Example:
Device(config-if)# exit
 

Exits interface configuration mode and enters global configuration mode.

 
Step 8 interface type number


Example:
Device(config)# interface gigabitethernet 0/0/0
 

Configures an interface and enters interface configuration mode.

 
Step 9 ip nat outside


Example:
Device(config-if)# ip nat outside
 

Indicates that the interface is connected to the outside network.

 
Step 10 end


Example:
Device(config-if)# end
 

Exits interface configuration mode and enters privileged EXEC mode.

 
Step 11 show ip nat translations [verbose]


Example:
Device# show ip nat translations
 

Displays active NAT translations.

 

Example

The following is sample output from the show ip nat translations command:

Device# show ip nat translations

Pro  Inside global         Inside local          Outside local         Outside global
udp  10.5.5.1:1025          192.0.2.1:4000        ---                   ---
udp  10.5.5.1:1024          192.0.2.3:4000        ---                   ---
udp  10.5.5.1:1026          192.0.2.2:4000        ---                   ---

Total number of translations: 3

The following is sample output from the show ip nat translations verbose command:

Device# show ip nat translations verbose

Pro  Inside global         Inside local          Outside local         Outside global
udp  10.5.5.1:1025          192.0.2.1:4000        ---                   ---
  create: 02/15/12 11:38:01, use: 02/15/12 11:39:02, timeout: 00:00:00
  Map-Id(In): 1
  Mac-Address: 0000.0000.0000    Input-IDB: TenGigabitEthernet1/1/0
  entry-id: 0x0, use_count:1

udp  10.5.5.1:1024          192.0.2.3:4000        ---                   ---
  create: 02/15/12 11:38:00, use: 02/15/12 11:39:02, timeout: 00:00:00
  Map-Id(In): 1
  Mac-Address: 0000.0000.0000    Input-IDB: TenGigabitEthernet1/1/0
  entry-id: 0x0, use_count:1

udp  10.5.5.1:1026          192.0.2.2:4000        ---                   ---
  create: 02/15/12 11:38:00, use: 02/15/12 11:39:02, timeout: 00:00:00
  Map-Id(In): 1
  Mac-Address: 0000.0000.0000    Input-IDB: TenGigabitEthernet1/1/0
  entry-id: 0x0, use_count:1

Total number of translations: 3

Configuring Dynamic Carrier Grade NAT

Dynamic address translation (dynamic NAT) maps unregistered IP addresses to registered IP addresses from a pool of registered IP addresses.

SUMMARY STEPS

1. enable

2. configure terminal

3. ip nat settings mode cgn

4. access-list standard-access-list-number permit source wildcard

5. access-list standard-access-list-number permit source wildcard

6. route-map map-tag

7. match ip address [access-list-number]

8. match ip next-hop [access-list-number]

9. exit

10. ip nat pool name start-ip end-ip prefix-length prefix-length

11. ip nat inside source route-map name pool name

12. interface gigabitethernet card/spaslot/port.subinterface-number

13. ip nat inside

14. exit

15. interface type number

16. ip nat outside

17. end


DETAILED STEPS
  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 nat settings mode cgn


Example:
Device(config)# ip nat settings mode cgn
 

Enables CGN operating mode.

 
Step 4 access-list standard-access-list-number permit source wildcard


Example:
Device(config)# access-list 1 permit 10.1.1.0 0.0.0.255
 
Defines a standard access list and specifies a host.
  • Access list 1 defined in this step is used by the match ip address command.

 
Step 5 access-list standard-access-list-number permit source wildcard


Example:
Device(config)# access-list 2 permit 10.5.5.0 0.0.0.255
 
Defines a standard access list and specifies a host.
  • Access list 2 defined in this step is used by the match ip next-hop command.

 
Step 6 route-map map-tag


Example:
Device(config)# route-map nat-route-map
 

Defines conditions for redistributing routes from one routing protocol into another or enables policy routing and enters route-map configuration mode.

 
Step 7 match ip address [access-list-number]


Example:
Device(config-route-map)# match ip address 1
 

Distributes any routes that have a destination network number address that is permitted by a standard access list, an extended access list, or a prefix list or performs policy routing on packets.

 
Step 8 match ip next-hop [access-list-number]


Example:
Device(config-route-map)# match ip next-hop 2
 

Redistributes any routes that have a next-hop router address passed by one of the specified access lists.

 
Step 9 exit


Example:
Device(config-route-map)# exit
 

Exits route-map configuration mode and enters global configuration mode.

 
Step 10 ip nat pool name start-ip end-ip prefix-length prefix-length


Example:
Device(config)# ip nat pool nat-pool 10.1.1.1 10.1.254.254 prefix-length 16
 

Defines a pool of IP addresses for NAT.

 
Step 11 ip nat inside source route-map name pool name


Example:
Device(config)# ip nat inside source route-map nat-route-map pool nat-pool
 

Enables dynamic NAT of the inside source address.

 
Step 12 interface gigabitethernet card/spaslot/port.subinterface-number


Example:
Device(config)# interface gigabitethernet 0/0/5
 

Configures an interface and enters interface configuration mode.

Note   

The NAT inside network can be applied to interface virtual-template when the router is used for broadband aggregation deployment.

 
Step 13 ip nat inside


Example:
Device(config-if)# ip nat inside
 

Indicates that the interface is connected to the inside network (the network that is subject to NAT translation).

 
Step 14 exit


Example:
Device(config-if)# exit
 

Exits interface configuration mode and enters global configuration mode.

 
Step 15 interface type number


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

Configures an interface and enters interface configuration mode.

 
Step 16 ip nat outside


Example:
Device(config-if)# ip nat outside
 

Indicates that the interface is connected to the outside network.

 
Step 17 end


Example:
Device(config-if)# end
 

Exits interface configuration mode and enters privileged EXEC mode.

 

Configuring Dynamic Port Address Carrier Grade NAT

Port Address Translation (PAT) or overloading is a form of dynamic NAT that maps multiple unregistered IP addresses to a single registered IP address (many-to-one mapping) by using different ports. PAT enables thousands of users to connect to the Internet by using only one real global IP address.

SUMMARY STEPS

1. enable

2. configure terminal

3. ip nat settings mode cgn

4. ip nat inside source list number pool name [overload]

5. ip nat pool name start-ip end-ip netmask netmask

6. access-list standard-access-list-number permit source wildcard

7. interface gigabitethernet card/spaslot/port.subinterface-number

8. ip nat inside

9. exit

10. interface type number

11. ip nat outside

12. end

13. show ip nat statistics


DETAILED STEPS
  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 nat settings mode cgn


Example:
Device(config)# ip nat settings mode cgn
 

Enables CGN operating mode.

 
Step 4 ip nat inside source list number pool name [overload]


Example:
Device(config)# ip nat inside source list 1 pool nat-pool overload
 
Enables the router to use one global address for many local addresses.
  • When you configure the overload keyword, the TCP or UDP port number of each inside host distinguishes between multiple conversations using the same local IP address.

  • The overload keyword configures overloading or PAT.

 
Step 5 ip nat pool name start-ip end-ip netmask netmask


Example:
Device(config)# ip nat pool nat-pool 10.1.1.1 10.1.254.254 netmask 255.255.0.0
 

Defines a pool of IP addresses for NAT.

 
Step 6 access-list standard-access-list-number permit source wildcard


Example:
Device(config)# access-list 1 permit 172.16.0.0 255.255.0.0
 

Defines a standard access list and specifies a host.

 
Step 7 interface gigabitethernet card/spaslot/port.subinterface-number


Example:
Device(config)# interface gigabitethernet 0/0/6
 

Configures an interface and enters interface configuration mode.

Note   

The NAT inside network can be applied to interface virtual-template when the router is used for broadband aggregation deployment.

 
Step 8 ip nat inside


Example:
Device(config-if)# ip nat inside
 

Indicates that the interface is connected to the inside network (the network that is subject to NAT translation).

 
Step 9 exit


Example:
Device(config-if)# exit
 

Exits interface configuration mode and enters global configuration mode.

 
Step 10 interface type number


Example:
Device(config)# interface gigabitethernet 0/0/2
 

Configures an interface and enters interface configuration mode.

 
Step 11 ip nat outside


Example:
Device(config-if)# ip nat outside
 

Indicates that the interface is connected to the outside network.

 
Step 12 end


Example:
Device(config-if)# end
 

Exits interface configuration mode and enters privileged EXEC mode.

 
Step 13 show ip nat statistics


Example:
Device# show ip nat statistics
 

Displays NAT statistics.

 

Example

The following is sample output from the show ip nat statistics command:

Device# show ip nat statistics

Total active translations: 3 (0 static, 3 dynamic; 3 extended)
Outside interfaces:
  TenGigabitEthernet2/0/0, TenGigabitEthernet2/1/0, TenGigabitEthernet2/2/0
  TenGigabitEthernet2/3/0
Inside interfaces: 
  TenGigabitEthernet1/0/0, TenGigabitEthernet1/1/0, TenGigabitEthernet1/2/0
  TenGigabitEthernet1/3/0
Hits: 59230465  Misses: 3
CEF Translated packets: 0, CEF Punted packets: 0
Expired translations: 0
Dynamic mappings:
-- Inside Source
[Id: 1] access-list 102 pool mypool refcount 3
 pool mypool: netmask 255.255.255.0
        start 10.5.5.1 end 10.5.5.5
        type generic, total addresses 5, allocated 1 (20%), misses 0
nat-limit statistics:
 max entry: max allowed 2147483647, used 3, missed 0
Pool stats drop: 0  Mapping stats drop: 0
Port block alloc fail: 0
IP alias add fail: 0
Limit entry add fail: 0

Configuration Examples for Carrier Grade Network Address Translation

Example: Configuring Static Carrier Grade NAT

Device# configure terminal
Device(config)# ip nat settings mode cgn
Device(config)# ip nat inside source static 192.168.2.1 192.168.34.2
Device(config)# interface gigabitethernet 0/0/6
Device(config-if)# ip nat inside
Device(config-if)# exit
Device(config)# interface gigabitethernet 0/0/0
Device(config-if)# ip nat outside
Device(config-if)# end 

Example: Configuring Dynamic Carrier Grade NAT

Device# configure terminal
Device(config)# ip nat settings mode cgn
Device(config)# access-list 1 permit 10.1.1.0 0.0.0.255
Device(config)# access-list 2 permit 10.5.5.0 0.0.0.255
Device(config)# route-map nat-route-map
Device(config-route-map)# match ip address 1
Device(config-route-map)# match ip next-hop 2
Device(config-route-map)# exit
Device(config)# ip nat pool nat-pool 10.1.1.1 10.1.254.254 prefix-length 16
Device(config)# ip nat inside source route-map nat-route-map pool nat-pool
Device(config)# interface gigabitethernet 0/0/5
Device(config-if)# ip nat inside
Device(config-if)# exit
Device(config)# interface gigabitethernet 0/0/1
Device(config-if)# ip nat outside
Device(config-if)# end

Example: Configuring Dynamic Port Address Carrier Grade NAT

Device# configure terminal
Device(config)# ip nat settings mode cgn
Device(config)# ip nat inside source list 1 pool nat-pool overload
Device(config)# ip nat pool nat-pool 10.1.1.1 10.1.254.254 netmask 255.255.0.0
Device(config)# access-list 1 permit 172.16.0.0 255.255.0.0
Device(config)# interface gigabitethernet 0/0/4
Device(config-if)# ip nat inside
Device(config-if)# exit
Device(config)# interface gigabitethernet 0/0/3
Device(config-if)# ip nat outside
Device(config-if)# end

Additional References for Carrier Grade Network Address Translation

Related Documents

Related Topic

Document Title

Cisco IOS commands

Master Command List, All Releases

NAT commands

IP Addressing Command Reference

NAT ALGs

“Using Application-Level Gateways with NAT”

HSL messages

“Monitoring and Maintaining NAT”

Standards and RFCs

Standard/RFC

Title

RFC 4787

Network Address Translation (NAT) Behavioral Requirements for Unicast UDP

RFC 5582

Location-to-URL Mapping Architecture and Framework

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Feature Information for Carrier Grade Network Address Translation

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/​go/​cfn. An account on Cisco.com is not required.
Table 1 Feature Information for Carrier Grade Network Address Translation

Feature Name

Releases

Feature Information

Carrier Grade Network Address Translation

Cisco IOS XE Release 3.6S

Carrier Grade Network Address Translation (CGN) is a large-scale NAT that translates private IPv4 addresses into public IPv4 addresses. CGN employs Network Address and Port Translation methods to aggregate multiple private IPv4 addresses into fewer public IPv4 addresses.

The following commands were introduced or modified: ip nat settings mode and ip nat settings support mapping outside.