Receiving A Leave Message - HP A6600 Configuration Manual

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 know 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 a forwarding table entry for the group address in the message exists, and, if one exists, whether
the outgoing port list contains the port.
• 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.
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.
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 it
through all its router ports in the VLAN and all member ports for that multicast group, and performs the
following operation to the port on which it received the IGMP leave message:
• 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.
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 proxyingYou 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. For more information
about the IGMP report suppression mechanism for hosts, see "Configuring
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IGMP."