1-4forwarding entry for the member port corresponding to the host from the forwarding table when its agingtimer expires.When an IGMPv2 or IGMPv3 host leaves a multicast group, the host sends an IGMP leave message tothe multicast router to announce that it has left the multicast group. When the switch receives agroup-specific IGMP leave group message on a member port, it first checks whether a forwarding tableentry for that group exists, and, if one exists, whether its outgoing port list contains that port.z If the forwarding table entry does not exist or if its outgoing port list does not contain the port, theswitch discards the IGMP leave group message instead of forwarding it to any port.z If the forwarding table entry exists and its outgoing port list contains the port, the switch forwardsthe IGMP leave group message to the router ports in the VLAN. Because the switch does not knowwhether any other member hosts for that group still exist under the port to which the leave messagearrived, the switch does not immediately remove the port from the outgoing port list; instead, theswitch resets the member port aging timer for that port.Upon receiving the IGMP leave group message from a host, the IGMP querier resolves from themessage the address of the multicast group that the host just left and sends an IGMP group-specificquery to that multicast group through the port that received the leave group message. Upon hearing theIGMP group-specific query, the switch forwards it through all its router ports in the VLAN and all memberports for that multicast group, and performs the following to the port (in case it is a dynamic member port)before the member port aging timer of the port expires:z If any IGMP report in response to the group-specific query is heard on a member port before itsaging timer expires, this means that some host attached to the port is receiving or expecting toreceive multicast data for that multicast group. The switch resets the aging timer of the memberport.z If no IGMP report in response to the group-specific query is heard on a member port before itsaging timer expires, this means that no hosts attached to the port are still listening to that groupaddress. The switch removes the port from the outgoing port list of the forwarding table entry forthat multicast group when the aging timer expires.Protocols and StandardsIGMP snooping is documented in:z RFC 4541: Considerations for Internet Group Management Protocol (IGMP) and Multicast ListenerDiscovery (MLD) Snooping SwitchesConfiguring IGMP SnoopingConfiguration Task ListPerform the tasks in Table 1-1 to configure IGMP snooping.Table 1-1 IGMP snooping configuration task listTask RemarksEnabling IGMP snoopingGloballyRequiredDisabled by default.
