Note	We recommend that you do not disable IGMP snooping on the switch. If you disable IGMP snooping, you may see reduced multicast performance because of excessive false flooding within the switch.

The Internet Group Management Protocol (IGMP) snooping software examines Layer 2 IP multicast traffic within a VLAN to discover the ports where interested receivers reside. Using the port information, IGMP snooping can reduce bandwidth consumption in a multi-access LAN environment to avoid flooding the entire VLAN. The IGMP snooping feature tracks which ports are attached to multicast-capable routers to help the routers forward IGMP membership reports. The IGMP snooping software responds to topology change notifications. By default, IGMP snooping is enabled on the switch.

The following figure shows an IGMP snooping switch that sits between the host and the IGMP router. The IGMP snooping switch snoops the IGMP membership reports and Leave messages and forwards them only when necessary to the connected IGMP routers.

The IGMP snooping software operates upon IGMPv1, IGMPv2, and IGMPv3 control plane packets where Layer 3 control plane packets are intercepted and influence the Layer 2 forwarding behavior.

For more information about IGMP, see Configuring IGMP.

The Cisco NX-OS IGMP snooping software has the following proprietary features:

• Source filtering that allows forwarding of multicast packets based on destination and source IP.

• Multicast forwarding based on IP address rather than MAC address.

• Optimized multicast flooding (OMF) that forwards unknown traffic to routers only and performs no data driven state creation.

For more information about IGMP snooping, see RFC 4541.

This section includes the following topics:

IGMP snooping has the following guidelines and limitations:

• IGMP snooping is not supported with PVLAN.

• When IGMPv3 host on a VLAN leaves, it is possible that other hosts may experience traffic drop. This is seen mostly when a second consecutive leave is received from the port that already left and this impacts the other receivers on the VLAN.

  To avoid this loss, you need to disable explicit host tracking under VLAN configuration using the no ip igmp snooping explicit-tracking command.

  For example:

  configure terminal
  vlan configuration 10
  no ip igmp snooping explicit-tracking
  

• In a hop-by-hop topology, the configuration of SVI on an intermediate box (second device) which is not an IGMP snooping querier causes traffic loss to hosts behind it when one of the other receivers ports behind another downstream L2 switch (third device) sends a leave. This is due to v3 suppression being disabled, IGMPv3 leave is consumed on second device. Workarounds for this issue is:

  • PIM DR and IGMP querier have to be co-located on the same box in the hop-by-hop topology. SVI in the first device should be configured with ip pim dr-priority 10 to shift the DR from second device to the firstt device and the default suppression should be disabled on the second device, third device, and so on.

  • IGMPV3 suppression should be enabled under the VLAN configuration for the impacted VLAN on all the hops such as the second device and the third device.

  For example:

  configure terminal
  vlan configuration 203
  ip igmp snooping v3-report-suppression
  

IGMP snooping has the following prerequisites:

• You are logged onto the switch.

• For global commands, you are in the correct virtual routing and forwarding (VRF) mode. The default configuration mode shown in the examples in this chapter applies to the default VRF.

To display the IGMP snooping configuration information, perform one of the following tasks:

For detailed information about the fields in the output from these commands, see the Cisco Nexus 3000 Series Multicast Routing Command Reference.

You can clear the IGMP snooping statistics using these commands.

This example shows how to configure the IGMP snooping parameters:

configure terminal
ip igmp snooping
vlan configuration 2
ip igmp snooping
ip igmp snooping explicit-tracking
ip igmp snooping fast-leave
ip igmp snooping last-member-query-interval 3
ip igmp snooping querier 172.20.52.106
ip igmp snooping mrouter interface ethernet 2/1
ip igmp snooping static-group 230.0.0.1 interface ethernet 2/1
ip igmp snooping link-local-groups-suppression

The following example shows how to configure prefix lists and use them to filter IGMP snooping reports:

ip prefix-list plist seq 5 permit 224.1.1.1/32
ip prefix-list plist seq 10 permit 224.1.1.2/32
ip prefix-list plist seq 15 deny 224.1.1.3/32
ip prefix-list plist seq 20 deny 225.0.0.0/8 eq 32
 
vlan configuration 2
ip igmp snooping report-policy prefix-list plist interface Ethernet 2/2
ip igmp snooping report-policy prefix-list plist interface Ethernet 2/3

In the above example, the prefix-list permits 224.1.1.1 and 224.1.1.2 but rejects 224.1.1.3 and all the groups in the 225.0.0.0/8 range. The prefix-list is an implicit "deny" if there is no match. If you wish to permit everything else, add ip prefix-list plist seq 30 permit 224.0.0.0/4 eq 32.

The following example shows how to configure route maps and use them to filter IGMP snooping reports:

route-map rmap permit 10
match ip multicast group 224.1.1.1/32
route-map rmap permit 20
match ip multicast group 224.1.1.2/32
route-map rmap deny 30
match ip multicast group 224.1.1.3/32
route-map rmap deny 40
match ip multicast group 225.0.0.0/8
 
vlan configuration 2
ip igmp snooping report-policy route-map rmap interface Ethernet 2/4
ip igmp snooping report-policy route-map rmap interface Ethernet 2/5

In the above example, the route-map permits 224.1.1.1 and 224.1.1.2 but rejects 224.1.1.3 and all the groups in the 225.0.0.0/8 range. The route-map is an implicit "deny" if there is no match. If you wish to permit everything else, add route-map rmap permit 50 match ip multicast group 224.0.0.0/4.

For additional information related to implementing IGMP snooping, see the following sections:

• Standards

• Related Documents