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that are monitoring the reported multicast address suppress their own reports. This makes the switch
unable to know whether the reported multicast group still has active members attached to that port.
When receiving a leave message
When an IGMPv1 host leaves a multicast group, the host does not send an IGMP leave message, and the
switch cannot know immediately that the host has left the multicast group. However, because the host
stops sending IGMP reports as soon as it leaves the multicast group, the switch removes the port that
connects to the host from the forwarding entry for the multicast group when the aging timer for the port
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 examines
whether a forwarding entry matches the group address in the message, and, if a match is found, whether
the forwarding entry for the group contains the dynamic member port.
•
If no forwarding entry matches the group address, or if the forwarding entry does not contain the
port, the switch directly discards the IGMP leave message.
•
If a forwarding entry matches the group address and the forwarding entry 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 forwarding entry for that group. 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 and 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 it through all its
router ports in the VLAN and all member ports of the multicast group. The switch also performs the
following judgment for the port that received the IGMP leave message:
•
If the port (assuming that it is a dynamic member port) receives an IGMP report in response to the
group-specific query before its aging timer expires, it indicates that some host attached to the port
is receiving or expecting to receive multicast data for the multicast group. The switch restarts the
aging timer for the port.
•
If the port receives no IGMP report in response to the group-specific query before its aging timer
expires, it indicates that no hosts attached to the port are still listening to that group address. The
switch removes the port from the forwarding entry for the multicast group when the aging timer
expires.
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.
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 take effect on it.
