Section 9-2. CGMP

9-2. CGMP

• CGMP is a cooperative protocol between Cisco routers and switches used to constrain multicast traffic.

• CGMP on a Catalyst switch relies on a CGMP router to provide it with multicast join or leave requests. This allows efficient multicast group registration without requiring the switch to inspect IGMP messages.

• CGMP uses destination multicast MAC address 01-00-0c-dd-dd-dd for all its messages. These messages are flooded to all switch ports so that even non-CGMP switches will relay CGMP information.

• CGMP requires that IGMP snooping be disabled. The two features are mutually exclusive. However, a router can be configured for both IGMP and CGMP to support a variety of switch platforms.

• CGMP is supported on all Catalyst switch platforms except the Catalyst 6000.

Configuration

1.

(Router only) Enable CGMP on a router:

IOS

 (interface) ip cgmp 

CGMP must be enabled on a router that also performs multicast routing. Once enabled, the router can send CGMP messages to Catalyst switches.

2.

Enable CGMP on the switch:

COS	set cgmp {enable | disable}
IOS	(global) [no] cgmp

By default, CGMP is disabled on COS switches but is enabled on IOS switches. Therefore, this command must be used on COS switches to enable CGMP.

3.

(Optional) Statically identify a CGMP router:

COS	set multicast router mod/port
IOS	N/A

By default, routers using CGMP announce themselves to CGMP switches. You can also give a static definition of a multicast router port if needed.

4.

(Optional) Use CGMP Fast-Leave:

COS	set cgmp leave {enable | disable}
IOS	(global) cgmp leave-processing

Fast-Leave is disabled by default. If enabled, the switch will listen to IGMPv2 leave group messages. When a leave message is intercepted on a port and there are no subsequent join messages, the port is pruned from the multicast group with no CGMP intervention from the router.

CGMP Example

First, a router is configured to support CGMP on its VLAN 199 interface, as follows:

IOS

 (global) interface vlan 199 (interface) ip cgmp 

A switch connected to VLAN 199 is configured to enable CGMP support. CGMP Fast-Leave Processing is also enabled for efficient handling of multicast leave requests, as follows:

COS	set cgmp enable set cgmp leave enable
IOS	(global) cgmp (global) cgmp leave-processing

Displaying Information About CGMP

Table 9-3 lists switch commands that you can use to display helpful information about CGMP.

Table 9-3. Switch Commands to Display CGMP Information

Display Function	Switch OS	Command
CGMP status	COS	show cgmp leave
	IOS	(exec) show cgmp state
CGMP statistics	COS	show cgmp statistics [vlan_id]
	IOS	N/A
CGMP routers	COS	show multicast router cgmp [mod/port] [vlan-id]
	IOS	(exec) show cgmp router [address]
CGMP groups	COS	show multicast group [mac-addr] [vlan-id]
	IOS	(exec) show cgmp {vlan vlan-id | group [address]}