9.6 Optional IP Multicast Routing Tasks
9.6.5
Configure CGMP
CGMP can run on an interface only if PIM is configured on the same interface. CGMP is disabled by default. To enable CGMP on the router, enter the following command in the interface configuration mode.

Router(config-if)#ip cgmp

This command enables the CGMP for IP multicast on a router and triggers a CGMP join message. The running configuration indicates if a specific router interface has been configured for CGMP. (see Figure )

When a no ip cgmp command is issued, a triggered CGMP leave message for Group 0000.0000.0000 is sent with the router MAC address on the interface.

Configuring CGMP on the switch allows IP multicast packets to be switched to only those ports that have IP multicast clients. Directing multicast traffic only to those user segments that have interested clients reduces the consumption of network bandwidth by not propagating IP multicast traffic throughout the broadcast domain. CGMP on a switch also reduces management and resource overhead by not requiring a separate VLAN for each multicast group in the switched network to separate traffic.

In order for CGMP to operate correctly on a switch, the switch must have a network connection to a router running CGMP. A CGMP-capable IP multicast router detects all IGMP packets and informs the switch when specific hosts join or leave IP multicast groups. When the CGMP-capable router receives an IGMP control packet, the router creates a CGMP packet that contains the request type, the multicast group address, and the actual MAC address of the host. The router then sends the CGMP packet to a well-known address to which all Catalyst® Series Switches listen.

CGMP on the switch automatically identifies the ports to which the CGMP-capable router is attached. However, static multicast router ports can be configured.

To enable CGMP on a set command-based switch, enter the following command in privileged EXEC mode.

Switch> (enable) set cgmp enable

Note: IGMP snooping must be disabled before you can enable CGMP.

The running configuration in Figure indicates if the specific switch has been configured for CGMP.

To disable CGMP on a switch, enter the set cgmp disable command. To enable CGMP and Fast Leave on a Cisco IOS command-based switch, enter the following command in global configuration mode:

Switch(config)#cgmp leave-processing

An example of this is shown in Figure .

To disable CGMP on the Cisco IOS command-based switch, issue the no form of the command.