IGMP Packet Forwarding Considerations

The snooping switch forwards received IGMP (Internet Group Management Protocol) Report and Leave messages only to multicast router-attached ports in that VLAN. IGMP queries are forwarded to all member interfaces of the VLAN.

The snooping switch is aware of link-layer changes caused by spanning tree operations. When a port is enabled or disabled by spanning tree, a general query is generated by the root bridge. This Topology Change Notification query is sent to all non-multicast router-attached ports of the root bridge, which aids in updating Layer 2 multicast forwarding entries faster so that network disruptions are felt only momentary.

The snooping switch processes all IGMP messages and drops invalid IGMP and MLD messages. Any unrecognized IGMP or MLD messages are forwarded in the VLAN. When the snooping switch originates an IGMP query (leave processing or TCN), it does not alter the version number or fields. The snooping switch leaves this information the same as the query information it received most recently on that VLAN.