3Com Switch 4800G 24-Port Configuration Manual page 556

556 C 42: IGMP S
HAPTER
n
C
NOOPING ONFIGURATION
When receiving a membership report
A host sends an IGMP report to the multicast router in the following
circumstances:
Upon receiving an IGMP query, a multicast group member host responds with
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an IGMP report.
When intended to join a multicast group, a host sends an IGMP report to the
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multicast router to announce that it is interested in the multicast information
addressed to that group.
Upon receiving an IGMP report, the switch forwards it through all the router ports
in the VLAN, resolves the address of the reported multicast group, and performs
the following:
If no forwarding table entry exists for the reported group, the switch creates an
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entry, adds the port as 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, but the port is not
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included in the outgoing port list for that group, 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
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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.
For the description of IGMP report suppression mechanism, refer to "Work
Mechanism of IGMPv1" on page 613.
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.
If the forwarding table entry does not exist or if its outgoing port list does not
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contain the port, the switch discards the IGMP leave group message instead of
forwarding it to any port.