3Com E4500-24 Cli Configuration Manual page 298

A switch will not forward an IGMP report through a non-router port for the following reason: Due to the
IGMP report suppression mechanism, if member hosts of that multicast group still exist under
non-router ports, the hosts will stop sending reports when they receive the message, and this prevents
the switch from knowing if members of that multicast group are still attached to these ports.
When receiving a leave message
When an IGMPv1 host leaves a multicast group, the host does not send an IGMP leave message, so
the switch cannot know immediately that the host has left the multicast group. However, as the host
stops sending IGMP reports as soon as it leaves a multicast group, the switch deletes the forwarding
entry for the member port corresponding to the host from the forwarding table when its aging timer
expires.
When an IGMPv2 or IGMPv3 host leaves a multicast group, the host sends an IGMP leave message to
the multicast router to announce that it has leaf the multicast group.
Upon receiving an IGMP leave message on the last member port, a switch forwards it out all router ports
in the VLAN. Because the switch does not know whether any other member hosts of that multicast
group still exists under the port to which the IGMP leave message arrived, the switch does not
immediately delete the forwarding entry corresponding to that port from the forwarding table; instead, it
resets the aging timer of the member port.
Upon receiving the IGMP leave 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 message. Upon receiving the IGMP
group-specific query, a switch forwards it through all the router ports in the VLAN and all member ports
of that multicast group, and performs the following to the receiving port:
If any IGMP report in response to the group-specific query arrives to the member port before its
aging timer expires, this means that some other members of that multicast group still exist under
that port: the switch resets the aging timer of the member port.
If no IGMP report in response to the group-specific query arrives to the member port before its
aging timer expires as a response to the IGMP group-specific query, this means that no members
of that multicast group still exist under the port: the switch deletes the forwarding entry
corresponding to the port from the forwarding table when the aging timer expires.
After an Ethernet switch enables IGMP Snooping, when it receives the IGMP leave message sent by a
host in a multicast group, it judges whether the multicast group exists automatically. If the multicast
group does not exist, the switch drops this IGMP leave message.
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