Published On: August 3ʳᵈ, 2020 06:05

IP Multicast Routing Configuration Guide, Cisco IOS XE Amsterdam 17.3.x (Catalyst 9500 Switches)

Restrictions for PIM Snooping

  • This feature is not supported on the C9500-12Q, C9500-16X, C9500-24Q, and C9500-40X models of the Cisco Catalyst 9500 Series Switches.

  • PIM snooping is supported only on IPv4 mroutes.

  • When PIM snooping is enabled and IGMP snooping is disabled in the VLAN, multicast packets are not bridged to the local receivers, within the VLAN, even after the local receivers send IGMP join request messages.

  • Directly connected sources are supported for bidirectional PIM groups. Traffic from directly connected sources is forwarded to the designated router and the designated forwarder for a VLAN. In some cases, a nondesignated router can receive a downstream (S, G) join message. For source-only networks, the initial unknown traffic is flooded only to the designated routers and designated forwarders.

  • All (S,G) mroutes are processed as (*,G) mroutes by the router.

  • Non-PIMv2 multicast routers will not receive traffic if PIM snooping is enabled.

Information About PIM Snooping

About PIM Snooping


Note

PIM snooping is disabled by default.


In networks where a Layer 2 switch interconnects several routers, such as an Internet exchange point (IXP), the switch floods IP multicast packets on all multicast router ports by default, even if there are no multicast receivers downstream. With PIM snooping enabled, the switch restricts multicast packets for each IP multicast group to only those multicast router ports that have downstream receivers joined to that group. When you enable PIM snooping, the switch learns which multicast router ports need to receive the multicast traffic within a specific VLAN by listening to the PIM hello messages, PIM join and prune messages, and bidirectional PIM designated forwarder election messages.


Note

We recommend that you use PIM snooping along with IGMP snooping on the switch. IGMP snooping restricts the multicast traffic that exits through the LAN ports to which the hosts are connected. However, IGMP snooping does not restrict traffic that exits through the LAN ports to which one or more multicast routers are connected.


The following illustrations show the flow of traffic and flooding that results in networks without PIM snooping enabled, and the flow of traffic and traffic restriction when PIM snooping is enabled.

Figure 1. PIM Join Message Flow without PIM Snooping. The following figure shows the flow of a PIM join message without PIM snooping enabled. In the figure, the switches flood the PIM join message, which is intended for Router B, to all the connected routers. Figure 2. PIM Join Message Flow with PIM Snooping. The following figure shows the flow of a PIM join message with PIM snooping enabled. In the figure, the switches restrict the PIM join message, and forward it only to the router that needs to receive it (Router B). Figure 3. Data Traffic Flow without PIM Snooping. The following figure shows the flow of data traffic without PIM snooping enabled. In the figure, the switches flood the data traffic, intended for Router A, to all the connected routers. Figure 4. Data Traffic Flow with PIM Snooping. The following figure shows the flow of data traffic with PIM snooping enabled. In the figure, the switches forward the data traffic only to the router that needs to receive it (Router A).

PIM Snooping on VLAN

The following characteristics are applicable if PIM Snooping is enabled on a VLAN:

  • PIM snooping can be enabled or disabled on a per-VLAN basis.

  • The switch snoops on designated forwarder election messages and maintains a list of all the designated forwarder routers for various RPs for a VLAN. All the traffic is sent to all the designated forwarders, which ensures that the bidirectional functionality works properly.

  • AUTO-RP groups (224.0.1.39 and 224.0.1.40) are always flooded on all the PIM router ports on all the PIM snooping-enabled VLANs.

  • All mroute state and neighbor information is maintained per VLAN.

  • Join or prune messages are not flooded on all the router ports, but are sent only to the port corresponding to the upstream router mentioned in the payload of the join or prune message.

  • When enabling the PIM sparse mode (PIM-SM) feature, downstream routers can view traffic only if the routers have previously indicated interest through a PIM join or prune message. An upstream router can only view traffic if used as an upstream router during the PIM join or prune process.

  • All mroute and router information is timed out based on the hold-time indicated in the PIM hello and join or prune message.

How to Configure PIM Snooping

Enabling PIM Snooping Globally


Note

You do not have to configure an IP address or IP PIM in order to run PIM snooping


To enable PIM snooping globally, perform this procedure:

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. ip pim snooping
  4. end
  5. show ip pim snooping

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 pim snooping

Example:

Device(config)# ip pim snooping

Enables PIM snooping.

Step 4

end

Example:

Device(config)# end

Returns to privileged EXEC mode.

Step 5

show ip pim snooping

Example:

Device# show ip pim snooping

Verifies your entries.

Enabling PIM Snooping in a VLAN

To enable PIM snooping in a VLAN, perform this procedure:

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. ip pim snooping vlan vlan_ID
  4. end
  5. show ip pim snooping vlan vlan_ID

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 pim snooping vlan vlan_ID

Example:

Device(config)# ip pim snooping vlan 10

Enables PIM snooping.

Step 4

end

Example:

Device(config)# end

Returns to privileged EXEC mode.

Step 5

show ip pim snooping vlan vlan_ID

Example:

Device# show ip pim snooping vlan 10

Verifies your entries.

Disabling PIM Snooping-Designated Router Flooding

By default, switches that have PIM snooping enabled will flood multicast traffic to the designated router. This method of operation can send unnecessary multicast packets to the designated router, which means that the network must carry unnecessary traffic, and the designated router must process and drop this traffic.

To reduce the traffic sent over the network to the designated router, disable designated router flooding. When designated router flooding is disabled, PIM snooping ensures that the designated router receives only the multicast traffic for which it has sent explicit join message.

To disable PIM snooping-designated router flooding, perform this procedure:

Before you begin

  • Do not disable designated router flooding on switches in a Layer 2 broadcast domain that supports multicast sources.

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. no ip pim snooping dr-flood
  4. 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

no ip pim snooping dr-flood

Example:

Device(config)# no ip pim snooping dr-flood

Disables PIM snooping-designated router flooding.

Step 4

end

Example:

Device(config)# end

Returns to privileged EXEC mode.

Monitoring PIM Snooping Information

Use the privileged EXEC commands in the following table to monitor your PIM snooping configurations.

Table 1. Commands to Monitor PIM Snooping

Command

Purpose

show ip pim snooping detail

Displays the operational state information.

show ip pim snooping vlan vlan_ID detail

Displays the operational state information of a VLAN.

show ip pim snooping mroute

Displays information about the mroute database.

show ip pim snooping vlan vlan_ID mroute

Displays information about the mroute of a VLAN.

show ip pim snooping neighbor

Displays information about the neighbor database.

show ip pim snooping vlan vlan_ID neighbor

Displays information about a VLAN's neighbor.

show ip pim snooping statistics

Displays information about VLAN statistics.

Configuration Examples for PIM Snooping

Example: Enabling PIM Snooping Globally

The following example shows how to enable PIM snooping globally and verify the configuration:


Device(config)#ip pim snooping
Device(config)#end
Device#show ip pim snooping
Global runtime mode: Enabled
Global admin mode : Enabled
DR Flooding status : Disabled
SGR-Prune Suppression: Enabled
Number of user enabled VLANs: 1
User enabled VLANs: 1001

Example: Enabling PIM Snooping in a VLAN

The following example shows how to enable PIM snooping on VLAN 1001 and verify the configuration:


Device(config)#ip pim snooping vlan 1001
Device(config)#end
Device#show ip pim snooping vlan 1001
4 neighbors (0 DR priority incapable, 4 Bi-dir incapable)
5000 mroutes, 0 mac entries
DR is 10.10.10.4
RP DF Set:
QinQ snooping : Disabled

Example: Disabling PIM Snooping-Designated Router Flooding

The following example shows how to disable PIM snooping-designated router flooding:


Device(config)#no ip pim snooping dr-flood
Device(config)#end

Additional References for PIM Snooping

Related Documents

Related Topic Document Title

Complete syntax and usage information about the commands used in this chapter.

See the IP Multicast Routing Commands section of the Command Reference (Catalyst 9500 Series Switches)

Cisco IOS commands

Cisco IOS Master Commands List, All Releases

Feature History and Information for PIM Snooping

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 2. Feature Information for PIM Snooping

Feature Name

Releases

Feature Information

PIM Snooping

Cisco IOS XE Fuji 16.8.1a

In networks where a Layer 2 switch interconnects several routers, such as an Internet exchange point (IXP), the switch floods IP multicast packets on all multicast router ports by default, even if there are no multicast receivers downstream. With PIM snooping enabled, the switch restricts multicast packets for each IP multicast group to only those multicast router ports that have downstream receivers joined to that group. When you enable PIM snooping, the switch learns which multicast router ports need to receive the multicast traffic within a specific VLAN by listening to the PIM hello messages, PIM join and prune messages, and bidirectional PIM designated forwarder election messages.