15
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If an entry in the forwarding table 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, the switch
resets the aging timer for that port.
NOTE:
A switch does not forward an IGMP report through a non-router port. This is because 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 receiving this report according to the IGMP
report suppression mechanism on them, and this will prevent the switch from knowing 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, so
the switch cannot determine 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 an entry in the forwarding table exists for the group address in the message, and, if
one exists, whether the outgoing port list contains the port.
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If the entry in the forwarding table 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 entry in the forwarding table exists and the outgoing port list contains the port, the switch
forwards the leave message to all router ports in the native VLAN. Because the switch cannot
determine whether any other hosts attached to the port are still monitoring that group address,
the switch does not immediately remove the port from the outgoing port list of the entry in the
forwarding table for that group. Instead, it resets the aging timer for the port.
After 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. After receiving the IGMP group-specific query, the switch
forwards the query 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:
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If any IGMP report in response to the group-specific query is received on the port—suppose it is
a dynamic member port—before its aging timer expires, this means that a host attached to the
port is receiving or expecting to receive multicast data for that multicast group. The switch
resets the aging timer of the port.
•
If no IGMP report in response to the group-specific query is received on the port before its aging
timer expires, this means that no hosts attached to the port are still monitoring that group
address. The switch removes the port from the outgoing port list of the entry in the forwarding
table for that multicast group when the aging timer expires.
IGMP snooping proxying
The IGMP snooping proxying function on an edge device reduces the number of IGMP reports and
leave messages sent to its upstream device. The device configured with IGMP snooping proxying is
called "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 take effect on it.