3Com WX3000 Series Operation Manual page 456

When receiving a general query
The IGMP querier periodically sends IGMP general queries to all hosts and routers on the local subnet
to find out whether active multicast group members exist on the subnet.
Upon receiving an IGMP general query, the device forwards it through all ports in the VLAN except the
receiving port and performs the following to the receiving port:
If the receiving port is a router port existing in its router port list, the device resets the aging timer of
this router port.
If the receiving port is not a router port existing in its router port list, the device adds it into its router
port list and sets an aging timer for this router port.
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 an IGMP report.
When intended to join a multicast group, a host sends an IGMP report to the multicast router to
announce that it is interested in the multicast information addressed to that group.
Upon receiving an IGMP report, the device forwards it through all the router ports in the VLAN, resolves
the address of the multicast group the host is interested in, and performs the following to the receiving
port:
If the port is already in the forwarding table, the device resets the member port aging timer of the
port.
If the port is not in the forwarding table, the device installs an entry for this port in the forwarding
table and starts the member port aging timer of this port.
A device 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 device 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 device 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 device 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 device forwards it out all router
ports in the VLAN. Because the device 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 device does not
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