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.

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 should double check that the feature is enabled on your switch. On many of these switch types, you enable CGMP by simply typing the command "cgmp" in global configuration mode. For example, on a Catalyst 1900:

Switch-1900#configure terminal Enter configuration commands, one per line. End with CNTL/Z Switch-1900(config)#cgmp Switch-1900(config)#exit Switch-1900#show cgmp CGMP Status : Enabled CGMP Fast Leave Status : Disabled CGMP Holdtime (secs) : 600 Allow Reserved Address to Join : Enabled VLAN Address Destination ----------------------------------------------------------------------- 1 0100.5E7F.FFFA Et0/22, Et0/24 1 0100.5E00.0128 Et0/22 VLAN Router Address Expiration Interface ----------------------------------------------------------------------- 1 0010.7B3B.27E5 595 sec Et0/22 Switch-1900#

Similarly, on a 3500XL, the commands are the same, although the output is slightly different:

Switch-3500XL#configure terminal Enter configuration commands, one per line. End with CNTL/Z Switch-3500XL(config)#cgmp Switch-3500XL(config)#exit Switch-3500XL#show cgmp CGMP is running. CGMP Fast Leave is not running. CGMP Allow reserved address to join GDA . Default router timeout is 300 sec. vLAN IGMP MAC Address Interfaces ------ ----------------- ----------- 1 0100.5e00.0128 Fa0/1 vLAN IGMP Router Expire Interface ------ ----------------- -------- ---------- 1 0009.7cb7.c9e2 288 sec Fa0/1 Switch-3500XL#