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removes the port from the associated forwarding entry. For a static member port, this mechanism
does not take effect.
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
examines whether a forwarding entry matches the group address in the message:.
•
If no match is found. the switch directly discards the IGMP leave message.
•
If a match is found but the receiving port is not in the forwarding entry, the switch directly
discards the IGMP leave message.
•
If a match is found and the receiving port is in the forwarding entry, the switch forwards the
leave message to all router ports in the VLAN. Without knowing whether any other attached
hosts are still listening to that group, the switch does not immediately remove the port from the
forwarding entry. Instead, it restarts the aging timer for the port.
After receiving the IGMP leave message, the IGMP querier resolves the multicast group address in
the message. Then, it sends an IGMP group-specific query to the multicast group through the port
that received the leave message.
After receiving the IGMP group-specific query, the switch forwards the query through all its router
ports in the VLAN and all member ports of the multicast group. Then, the switch waits for the
responding IGMP reports from the directly connected hosts to check for the existence of members
for the multicast group. For the port that receives the leave message (assuming that it is a dynamic
member port), the Layer 2 device also performs one of the following actions:
•
If the port receives an IGMP report before the aging timer expires, the switch restarts the aging
timer for the port.
•
If the port does not receive an IGMP report when the aging timer expires, the switch removes
the port from the forwarding entry for the multicast group.
IGMP snooping proxying
You can configure the IGMP snooping proxying function on an edge device to reduce the number of
IGMP reports and leave messages sent to its upstream device. The device configured with IGMP
snooping proxying is called an IGMP snooping proxy. It is a host from the perspective of its upstream
device.
NOTE:
Even though an IGMP snooping proxy is a host from the perspective of its upstream device, the
IGMP membership report suppression mechanism for hosts does not affect it. For more information
about the IGMP report suppression mechanism for hosts, see "
