Operation Manual – Multicast Protocol
H3C S3610&S5510 Series Ethernet Switches
Chapter 2 IGMP Snooping Configuration
2-5
Note:
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
III. 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.
z
If the forwarding table entry does not exist or if its outgoing port list does not
contain the port, the switch discards the IGMP leave group message instead of
forwarding it to any port.
z
If the forwarding table entry exists and its outgoing port list contains the port, the
switch forwards the leave group message to all router ports in the VLAN. Because
the switch does not know whether any other hosts attached to the port are still
listening to that group address, the switch does not immediately removes the port
from the outgoing port list of the forwarding table entry for that group; instead, it
resets the member port aging timer for the port.
Upon receiving the IGMP leave group 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 group message. Upon hearing the IGMP group-specific query, the switch
forwards it through all its router ports in the VLAN and all member ports for that
multicast group, and performs the following:
z
If any IGMP report in response to the group-specific query is heard on a member
port before its aging timer expires, this means that some host attached to the port
