H3C Technologies H3C S7500E Series Switches - Manual (Page 38)

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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 message exists, and, if
one exists, whether the outgoing port list contains the port.
z 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.
z 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 native 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 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:
z 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 some 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.
z 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 listening to that
group address: the switch removes the port from the outgoing port list of the forwarding
table entry for that multicast group when the aging timer expires.
IGMP Snooping Proxying
You can configure the IGMP snooping proxying function on an edge device, which then can
represent its attached hosts to send membership reports and leave messages, thus reducing
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. For the
IGMP report suppression mechanism for hosts, refer to IGMP Configuration in the IP Multicast
Configuration Guide.