H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 176)

24-4
If a forwarding table entry exists for the reported group, but the port is not included in the outgoing
port list, the switch adds the port as a member port to the outgoing port list, and starts a member
port aging timer for that port.
If a forwarding table entry exists for the reported group and the port is not included in the outgoing port
list, the switch resets the member port aging timer for that port.
A switch does not forward an IGMP report through a non-router port. This is because 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 according to the IGMP report
suppression mechanism on them, 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 message, so
the switch cannot know immediately that the host has left the multicast group. However, as the host
stops sending IGMP membership 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 left the multicast group. When the switch receives 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.
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.
If the forwarding table entry exists and its outgoing port list contains the port, the switch forwards
the IGMP leave group message to the router ports in the VLAN. Because the switch does not know
whether any other member hosts for that group still exist under the port to which the leave message
arrived, the switch does not immediately remove the port from the outgoing port list; instead, the
switch resets the member port aging timer for that 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 to the port (in case it is a dynamic member port)
before the member port aging timer of the port expires:
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.