IGMP Snooping

IGMP snooping is a Layer 2 function of the switch; it does not require multicast routing to be enabled. In IGMP snooping, the Layer 2 switch keeps track of IGMP reports and only forwards multicast traffic to that part of the local network that requires it. IGMP snooping optimizes the use of network bandwidth and prevents multicast traffic from being flooded to parts of the local network that do not need it. The switch does not reduce any IP multicast traffic in the local multicast domain (224.0.0.x).

IGMP snooping is enabled by default on all VLANs and VMANs in the switch. If IGMP snooping is disabled on a VLAN or VMAN, all IGMP and IP multicast traffic floods within the VLAN or VMAN. IGMP snooping expects at least one device on every VLAN to periodically generate IGMP query messages.

To enable or disable IGMP snooping, use the following command:

enable igmp snooping {forward-mcrouter-only | {vlan} name | with-proxy vr vrname}

disable igmp snooping {forward-mcrouter-only | {vlan} name | with-proxy vr vrname}

Note

IGMP snooping is not supported on SVLANs on any platform.

The IGMP snooping proxy feature represented by "with-proxy" in the above commands is enabled by default. This feature optimizes the forwarding of IGMPv1 and IGMPv2 reports. The following is true for each group:

Only the first received IGMP join is forwarded upstream.
Only the IGMP leave for last host is forwarded upstream.

When a switch receives an IGMP leave message on a port, it sends a group-specific query on that port if proxy is enabled (even if it is a non-querier). The switch removes the port from the group after leave timeout (The timeout value is configurable, with a default value of 1000 ms., and a range from 0 to 175000 ms). If all the ports are removed from the group, the group is deleted and the IGMP leave is forwarded upstream. If IGMP snooping proxy is disabled, then all the IGMP reports are forwarded upstream.

Note

IGMP snooping proxy does not apply to IGMPv3 reports.

IGMP snooping is implemented primarily through ACLs, which are processed on the interfaces. These special purpose ACLs are called IGMP snooping hardware filters. The software allows you to choose between two types of IGMP snooping hardware filters: per-port filters (the default) and per-VLAN filters.

The two types of IGMP snooping hardware filters use switch hardware resources in different ways. The two primary hardware resources to consider when selecting the IGMP snooping hardware filters are the Layer 3 multicast forwarding table, and the interface ACLs. The size of both of these hardware resources is determined by the switch model. In general, the per-port filters consume more resources from the multicast table and less resources from the ACL table. The per-VLAN filters consume less space from the multicast table, and more from the ACL table.

Using the per-port filters can fill up the multicast table and place an extra load on the CPU. To avoid this, configure the switch to use the per-VLAN filters.

Note

The impact of the per-VLAN filters on the ACL table increases with the number of VLANs configured on the switch. If you have a large number of configured VLANs, we suggest that you use the per-port filters.