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.