3Com 4210 PWR Configuration Manual page 201

Upon receiving an IGMP general query, the switch 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 switch
■
resets the aging timer of this router port.
If the receiving port is not a router port existing in its router port list, the switch
■
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 switch 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 switch resets the member port
■
aging timer of the port.
If the port is not in the forwarding table, the switch installs an entry for this
■
port in the forwarding table and starts the member port aging timer of this
port.
n
A switch 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 switch 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 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 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 switch
forwards it out all router ports in the VLAN. Because the switch 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 switch does not immediately delete
IGMP Snooping Overview 199