9.6 Optional IP Multicast Routing Tasks
9.6.6
Configure CGMP leave
The CGMP/IGMP-capable router sends periodic multicast group queries. If a host wants to remain in a multicast group, the host responds to the query from the router. If a host does not want to remain in the multicast group, the host does not respond to the router query.

If, after a number of queries, the router receives no reports from any host in a multicast group, the router sends a CGMP/IGMP command to the switch, telling it to remove the multicast group from its forwarding tables. The router does not send a CGMP/IGMP command to remove the multicast group until all the hosts in the group ask to leave the group.

CGMP fast-leave processing allows the switch to detect IGMPv2 leave messages. When the switch receives a leave message, the switch starts a query-response timer. If this timer expires before a CGMP join message is received, the port is pruned from the multicast tree for the multicast group specified in the original leave message. Fast-leave processing ensures optimal bandwidth management for all hosts on a switched network, even when multiple multicast groups are in use simultaneously.

To enable the CGMP leave function on the switch, enter the following command in privileged EXEC mode.

Switch> (enable) set cgmp leave

The show cgmp leave command provides verification that CGMP fast leave has been configured. An example of this is shown in the Figure.

To disable the CGMP fast-leave function, enter the set cgmp leave disable command.