Release 15.5SY Supervisor Engine 6T Software Configuration Guide

PIM Snooping

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• Prerequisites for PIM Snooping
• Restrictions for PIM Snooping
• Information About PIM Snooping
• Default Settings for PIM Snooping
• How to Configure PIM Snooping

Note ● For complete syntax and usage information for the commands used in this chapter, see these publications:

http://www.cisco.com/en/US/products/ps11846/prod_command_reference_list.html

• Cisco IOS Release 15.4SY supports only Ethernet interfaces. Cisco IOS Release 15.4SY does not support any WAN features or commands.

http://www.cisco.com/en/US/products/hw/switches/ps708/tsd_products_support_series_home.html

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Prerequisites for PIM Snooping

None.

Restrictions for PIM Snooping

• Multicast packets are not bridged in a VLAN to local receivers that send IGMP joins when PIM snooping is enabled in the VLAN and IGMP snooping is disabled in the VLAN. ( CSCta03980)
• When you use the PIM-sparse mode (PIM-SM) feature, downstream routers only see traffic if they previously indicated interest through a PIM join or prune message. An upstream router only sees traffic if it was used as an upstream router during the PIM join or prune process.
• Join or prune messages are not flooded on all router ports but are sent only to the port corresponding to the upstream router mentioned in the payload of the join or prune message.
• Directly connected sources are supported for bidirectional PIM groups. Traffic from directly connected sources is forwarded to the designated router and designated forwarder for a VLAN. In some cases, a nondesignated router (NDR) can receive a downstream (S, G) join. For source-only networks, the initial unknown traffic is flooded only to the designated routers and designated forwarders.
• Dense group mode traffic is seen as unknown traffic and is dropped.
• The AUTO-RP groups (224.0.1.39 and 224.0.1.40) are always flooded.
• The switch snoops on designated forwarder election and maintains a list of all designated forwarder routers for various RPs for the VLAN. All traffic is sent to all designated forwarders which ensures that bidirectional functionality works properly.
• PIM snooping and IGMP snooping can be enabled at the same time in a VLAN. Either RGMP or PIM snooping can be enabled in a VLAN but not both.
• Any non-PIMv2 multicast router will receive all traffic.
• You can enable or disable PIM snooping on a per-VLAN basis.
• All mroute and router information is timed out based on the hold-time indicated in the PIM hello and join/prune control packets. All mroute state and neighbor information is maintained per VLAN.
• With Release 15.1(1)SY1 and later releases, IGMP snooping and PIM snooping constrain VPLS multicast traffic.

Information About PIM Snooping

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 To use PIM snooping, you must enable IGMP snooping on the switch. IGMP snooping restricts multicast traffic that exits through the LAN ports to which hosts are connected. 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 8-1 shows the flow of a PIM join message without PIM snooping enabled. In the figure, the switches flood the PIM join message intended for Router B to all connected routers.

Figure 8-1 PIM Join Message Flow without PIM Snooping

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Figure 8-2 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 8-2 PIM Join Message Flow with PIM Snooping

Figure 8-3 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 connected routers.

Figure 8-3 Data Traffic Flow without PIM Snooping

Figure 8-4 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).

Figure 8-4 Data Traffic Flow with PIM Snooping

Default Settings for PIM Snooping

PIM snooping is disabled by default.

How to Configure PIM Snooping

• Enabling PIM Snooping Globally
• Enabling PIM Snooping in a VLAN
• Disabling PIM Snooping Designated-Router Flooding

Enabling PIM Snooping Globally

To enable PIM snooping globally, perform this task:

 

	Command	Purpose
Step 1	Router(config)# ip pim snooping	Enables PIM snooping.
Step 2	Router(config)# end	Exits configuration mode.

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

Router(config)# ip pim snooping

Router(config)# end

Router# show ip pim snooping

Global runtime mode: Enabled

Global admin mode : Enabled

Number of user enabled VLANs: 1

User enabled VLANs: 10

Router#

 

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Note You do not need to configure an IP address or IP PIM in order to run PIM snooping.

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Disabling PIM Snooping Designated-Router Flooding

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

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

To reduce the traffic sent over the network to the designated router, disable designated-router flooding. With designated-router flooding disabled, PIM snooping only passes to the designated-router traffic that is in multicast groups for which PIM snooping receives an explicit join from the link towards the designated router.

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

 

	Command	Purpose
Step 1	Router(config)# no ip pim snooping dr-flood	Disables PIM snooping designated-router flooding.
Step 2	Router(config)# end	Exits configuration mode.

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

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

Router(config)# end

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Tip For additional information about Cisco Catalyst 6500 Series Switches (including configuration examples and troubleshooting information), see the documents listed on this page:

http://www.cisco.com/en/US/products/hw/switches/ps708/tsd_products_support_series_home.html

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