After receiving the IGMP leave group message from a host, the IGMP querier resolves from the message the address of the multicast group that the host just left and sends an IGMP group-specific query to that multicast group through the port that received the leave group message. After hearing the IGMP group-specific query, the switch forwards it through all its router ports in the VLAN and all member ports for that multicast group, and performs the following to the port before the member port aging timer of the port expires (in case it is a dynamic member port):

∙If any IGMP report in response to the group-specific query is heard on a member port before its aging timer expires, this means that some host attached to the port is receiving or expecting to receive multicast data for that multicast group. The switch resets the aging timer of the member port.

∙If no IGMP report in response to the group-specific query is heard on a member port before its aging timer expires, this means that no hosts attached to the port are still listening to that group address. The switch removes the port from the outgoing port list of the forwarding table entry for that multicast group when the aging timer expires.

IGMP snooping querier

In an IP multicast network running IGMP, a multicast router or Layer 3 multicast switch is responsible for sending IGMP general queries, so that all Layer 3 multicast devices can establish and maintain multicast forwarding entries, thus to forward multicast traffic correctly at the network layer. This router or Layer 3 switch is called an IGMP querier.

However, a Layer 2 multicast switch does not support IGMP, and therefore cannot send general queries by default. By enabling IGMP snooping on a Layer 2 switch in a VLAN where multicast traffic needs to be Layer-2 switched only and no multicast routers are present, the Layer 2 switch will act as the IGMP snooping querier to send IGMP queries, thus allowing multicast forwarding entries to be established and maintained at the data link layer.

Protocols and standards

∙RFC 4541, Considerations for Internet Group Management Protocol (IGMP) and Multicast Listener Discovery (MLD) Snooping Switches

Configuring IGMP snooping

Configuration task list

Task

Remarks

 

 

Enabling IGMP snooping

Required

globally

Disabled by default.

 

 

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