2-5does not immediately remove the port from the outgoing port list of the forwarding table entry forthat group; instead, it resets the aging timer for the port.Upon receiving the IGMP leave message from a host, the IGMP querier resolves the multicast groupaddress in the message and sends an IGMP group-specific query to that multicast group through theport that received the leave message. Upon receiving the IGMP group-specific query, the switchforwards it through all its router ports in the VLAN and all member ports for that multicast group, andperforms the following to the port on which it received the IGMP leave message:z If any IGMP report in response to the group-specific query is received on the port (suppose it is adynamic member port) before its aging timer expires, this means that some host attached to theport is receiving or expecting to receive multicast data for that multicast group. The switch resetsthe aging timer of the port.z If no IGMP report in response to the group-specific query is received on the port before its agingtimer 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 thatmulticast group when the aging timer expires.IGMP Snooping ProxyingYou can configure the IGMP snooping proxying function on an edge device, which then can representits attached hosts to send membership reports and leave messages, thus reducing the number of IGMPreports and leave messages sent to its upstream device. The device configured with IGMP snoopingproxying is called an IGMP snooping proxy. It is a host from the perspective of its upstream device.Even though an IGMP snooping proxy is a host from the perspective of its upstream device, the IGMPmembership report suppression mechanism for hosts does not take effect on it.Figure 2-3 Network diagram for IGMP snooping proxyingAs shown in Figure 2-3, Switch A works as an IGMP Snooping proxy. It represents its attached hosts tosend membership reports and leave messages to Router A.
