Cisco IOS Cookbook, 2nd Edition

Configuring CGMP

Problem

You want the router to use CGMP to communicate with a Catalyst switch.

Solution

When you enable multicast routing and turn on PIM on an interface, IGMP is enabled by default. However, you must explicitly enable CGMP on the router if you want your Catalyst switch to take advantage of this efficient way of handling group membership:

Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#ip multicast-routing
Router1(config)#interface FastEthernet0/0
Router1(config-if)#ip pim sparse-dense-mode
Router1(config-if)#ip cgmp
Router1(config-if)#end
Router1#

Discussion

The introduction to this chapter discusses the reasons for CGMP and roughly how it works. It is important to remember that CGMP is a Cisco proprietary protocol, so it will only work with Catalyst switches. Table 23-1 shows the minimum switch software revision level required for CGMP for several types of switches. If in doubt, please check your switch’s documentation.

Table 23-1. Catalyst switches that support CGMP

Catalyst model	Minimum software version
1900	6.0
2820	6.0
2900XL	11.2(8)SA
2901, 2902, 2926T/F/G	2.3
2940	Not supported
2948G	All
2950/2955	Not supported
2970	Not supported
3500XL	11.2(8)SA
3550, 3560, 3750	Not supported
4000 series (CatOS)	All
4000 series (IOS)	Not supported
4912	All
5000	2.3
6000	Not supported

The syntax for enabling CGMP on these devices varies. Also, on some of the supported switches, CGMP is enabled by default while it is disabled by default on others. So you ...