IGMP snooping

VSX switches can be configured for IGMP snooping on downstream VLANs facing the access switches. When enabled, the IGMP group database is independently constructed on each VSX switch. Multicast traffic to these groups is appropriately pruned/optimized.

Each VSX switch has an identical IGMP group database:

  • Each VSX node individually learns any JOIN/LEAVE message received from a downstream VSX LAG.

    For example: Agg-1 learns on downlink from SW1, whereas Agg-2 learns on the ISL as the ISL is always included as a forwarding port for IGMP, as shown in the following figure.

  • The VSX IGMP process translates the received IGMP from the ISL into an IGMP join message from the VSX LAG.

Multicast traffic to these IGMP groups is pruned/forwarded based on the individual IGMP group database on each VSX node. ISLP does not synchronize IGMP groups between VSX peers. The IGMP database construction is a data-plane based process.

If a VSX node reboots, it must relearn all the IGMP groups. The VSX switch floods multicast traffic within the VLANs that have active physical ports being forwarded. It then sends an All Hosts Query message. When the VSX node receives all join messages, it relearns and recreates the IGMP groups database.

IGMP snooping