Cisco CCNP / BCMSN Exam Tutorial: Configuring CGMP On Routers & Switches
If a Layer Two switch doesn’t have the capabilities to run IGMP Snooping, it will be able to run CGMP – Cisco Group Membership Protocol. CGMP allows the multicast router to work with the Layer Two switch to eliminate unnecessary multicast forwarding.
CGMP will be enabled on both the multicast router and the switch, but the router’s going to do all the work. The router will be sending Join and Leave messages to the switch as needed. PIM must be running on the router interface facing the switch before enabling CGMP, as you can see:
R1(config)#int e0
R1(config-if)#ip cgmp
WARNING: CGMP requires PIM enabled on interface
R1(config-if)#ip pim sparse
R1(config-if)#ip cgmp
When CGMP is first enabled on both the multicast router and switch, the router will send a CGMP Join message, informing the switch that a multicast router is now connected to it. This particular CGMP Join will contain a Group Destination Address (GDA) of 0000.0000.0000 and the MAC address of the sending interface. The GDA is used to identify the multicast...
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