IGMP Snooping and Proxies

Without a proxy arrangement, IGMP snooping does not generate or introduce queries and reports. It will only “snoop” reports received from all of its interfaces (including multicast-router interfaces) to build its state and group (S,G) database.

Without a proxy, IGMP messages are processed as follows:

• Query—All general and group-specific IGMP query messages received on a multicast-router interface will be forwarded to all other interfaces (both multicast-router interfaces and host-side interfaces) on the bridge.
• Report—IGMP reports received on any interface of the bridge are forwarded towards other multicast-router interfaces. The receiving interface is added as an interface for that group if a multicast routing entry exists for this group. Also, a group timer is set for the group on that interface. If this timer expires (that is, there was no report for this group during the IGMP group timer period), then the interface is removed as an interface for that group.
• Leave—Any IGMP leave message received on any interface of the bridge.

Proxy snooping reduces the number of IGMP reports sent towards an IGMP router.

Note: With proxy snooping configured, an IGMP router is not able to perform host tracking.

As proxy for its host-side interfaces, IGMP snooping in proxy mode replies to the queries it receives from an IGMP router on a multicast-router interface. On the host-side interfaces, IGMP snooping in proxy mode behaves as an IGMP router and sends general and group-specific queries on those interfaces.

Note: Only group-specific queries are generated by IGMP snooping directly; general queries received from the multicast-router interfaces are flooded to host-side interfaces.

All the queries generated by IGMP snooping are sent using 0.0.0.0 as source address. Also, all reports generated by IGMP snooping are sent with 0.0.0.0 as source address unless there is a configured source address to use.

Proxy mode functions differently on multicast-router interfaces than it does on host-side interfaces.