R3303-HP HSR6800 Routers IP Multicast Configuration Guide

know whether the reported multicast group still has active members attached to that port. For more

information about the IGMP report suppression mechanism, see "Configuring IGMP."

When receiving a leave message

An IGMPv1 host silently leaves a multicast group and the router is not notified of the leave. However,

because the host stops sending IGMP reports as soon as it leaves the multicast group, the router removes

the port that connects to the host from the forwarding entry for the multicast group when the aging timer

for the port expires.

An IGMPv2 or IGMPv3 host sends an IGMP leave message to the multicast router when leaving a

multicast group.

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 means 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 means 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."