1-5When an IGMPv2 or IGMPv3 host leaves a multicast group, the host sends an IGMP leave message tothe multicast router.When the switch receives an IGMP leave message on a dynamic member port, the switch first checkswhether a forwarding table entry for the group address in the message exists, and, if one exists,whether the outgoing port list contains the port.z If the forwarding table entry does not exist or if the outgoing port list does not contain the port, theswitch discards the IGMP leave message instead of forwarding it to any port.z If the forwarding table entry exists and the outgoing port list contains the port, the switch forwardsthe leave message to all router ports in the native VLAN. Because the switch does not knowwhether any other hosts attached to the port are still listening to that group address, the switchdoes 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.
