H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 322)

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If a forwarding table entry exists for the reported group and the port is included in the outgoing port
list, which means that this port is already a member port, the switch resets the member port aging
timer for that port.
A switch does not forward an IGMP report through a non-router port. The reason is as follows: Due to
the IGMP report suppression mechanism, if the switch forwards a report message through a member
port, all the attached hosts listening to the reported multicast address will suppress their own reports
upon hearing this report, and this will prevent the switch from knowing whether any hosts attached to
that port are still active members of the reported multicast group.
When receiving a leave group message
When an IGMPv1 host leaves a multicast group, the host does not send an IGMP leave group 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 group
message to the multicast router.
When the switch hears a group-specific IGMP leave group message on a member port, it first checks
whether a forwarding table entry for that group exists, and, if one exists, whether its outgoing port list
contains that port.
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If the forwarding table entry does not exist or if its outgoing port list does not contain the port, the
switch discards the IGMP leave group message instead of forwarding it to any port.
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If the forwarding table entry exists and its outgoing port list contains the port, the switch forwards
the leave group message to all router ports in the VLAN. Because the switch does not know
whether any other hosts attached to the port are still listening to that group address, the switch
does not immediately removes the port from the outgoing port list of the forwarding table entry for
that group; instead, it resets the member port aging timer for the port.
Upon 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. Upon 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:
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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.
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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.