R3102-R3103-HP 6600/HSR6600 Routers IP Multicast Configuration Guide

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When the router receives an IGMP leave message on a dynamic member port, the router 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 router directly discards the IGMP leave message.
• If a forwarding entry matches the group address and the forwarding entry contains the port, the
router forwards the leave message to all router ports in the VLAN. Because the router does not know
whether any other hosts attached to the port are still listening to that group address, the router 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 router forwards it through all its
router ports in the VLAN and all member ports of the multicast group. The router 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 router 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
router 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 affect it. For more information about the
IGMP report suppression mechanism for hosts, see "Configuring IGMP."
Figure 13 Network diagram
Host A
Receiver
Host B
Host C
Receiver
IGMP Querier
Router A
Proxy & Querier
Router B
Query from Router A
Report from Switch A
Query from Switch A
Report from Host
IP network