3Com S7902E, Configuration Manual

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1-5
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 receiving this report according to the IGMP
report suppression mechanism on them, and this will prevent the switch from knowing whether the
reported multicast group still has active members attached to that port.
For the IGMP report suppression mechanism on hosts, refer to IGMP Configuration in the IP Multicast
Volume .
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 dynamic 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.
When the switch receives an IGMP leave message on a dynamic member port, the switch first checks
whether 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, the
switch 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 forwards
the leave message to all router ports in the native 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 remove the port from the outgoing port list of the forwarding table entry for
that 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 group
address in the message 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, 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 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 a
dynamic 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 port.
z
If no IGMP report in response to the group-specific query is received on the 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.