H3C s3100-52P Operation Manual page 24

Operation Manual – Multicast
H3C S3100-52P Ethernet Switch
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.
Note:
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.
III. 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 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 forwards it
Chapter 3 IGMP Snooping Configuration
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