3Com Switch 8800 Advanced Software V5 Configuration Guide

542 CHAPTER 41: IGMP SNOOPING CONFIGURATION
■ 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.
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 left the
multicast group.
Upon receiving an IGMP leave message on a member port, a switch forwards it to
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 the forwarding
entry corresponding to that port from the forwarding table; instead, it resets the
aging timer of the member port.
Upon receiving the IGMP leave 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 message. Upon receiving the IGMP group-specific query, a
switch (non-querier) forwards it through all the router ports in the VLAN and all
member ports of that multicast group, and performs the following to the receiving
port:
■ If any IGMP membership report in response to the group-specific query arrives
to the member port before its aging timer expires, this means that some other
members of that multicast group still exist under that port: the switch resets
the aging timer of the member port.
■ If no IGMP membership report in response to the group-specific query arrives
to this member port before its aging timer expires as a response to the IGMP
group-specific query, this means that no members of that multicast group still