H3C S9500 Series Operation Manual page 7

Operation Manual – IGMP Snooping
H3C S9500 Series Routing Switches
If a forwarding table entry exists for the reported group, but the port is not included
in the outgoing port list for that group in the forwarding entry, the switch adds the
port as a dynamic member port into the outgoing port list, and starts an aging timer
for that port.
If a forwarding table entry exists for the reported group and the port is included in
the outgoing port list, which means that this port is already a dynamic member port
for that group, the switch resets the aging timer for that port.
Note:
A switch will not forward an IGMP report through a non-router port.
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 dynamic 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.
When the switch receives an IGMP leave message on a dynamic member port, the
switch first checks whether a forwarding table entry for the group address in the leave
message exists, and, if one exists, whether the outgoing port list contains that port.
If the forwarding table entry does not exist or if the outgoing port list does not
contain the port, the switch discards the IGMP leave message instead of
forwarding it to any port.
If the forwarding table entry exists and the outgoing port list contains the port, the
switch forwards the leave 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 remove the port
from the outgoing port list of the forwarding table entry for that group; instead, it
resets the aging timer for the port.
Upon receiving the IGMP leave message from a host, the IGMP querier resolves the
multicast group address in the message 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, the switch (non-IGMP querier) forwards it through all its
router ports in the VLAN and all member ports for that multicast group, and performs
the following to the port on which it received the IGMP leave message:
Chapter 1 IGMP Snooping Configuration
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