cgmp is an alternative to igmp-snooping.
The essence of this by a switch is to bind the interested propriate multicast group to the appropriate switchport.
cgmp needs assistance by the router and hence it runs both on the switch and the router.
for eg. when a host sends an igmp report to the router requesting it to be added to the multicast group as a client, the cgmp enabled router sends a cgmp join message to the switch with the mac-address of the igmp report sender and the multicast group that was requested for in the igmp report msg.
Hence CGMP is required for multicast at layer2 and assisted by the router.
At the router, cgmp needs to be enabled at the interface level.
And, cgmp and igmp-snooping are mutually exclusive.
hope this helps
cheers
Arav
