I hope someone will be able to explain when / when not to use CGMP on a router.
Our router has multicast routing enabled, PIM-SM in place on both WAN and LAN interfaces. My understanding is that CGMP needs to be enabled on the router's LAN interface with IGMP Snooping enabled on the switch that connects the router's LAN interface.
Is CGMP needed on the router at all?
What is the purpose of CGMP as opposed to IGMP Snooping?
CGMP needs to be enabled on the router only if CGMP is used on the switches.
CGMP requires a one way communication from router to switches to signal ports to multicast groups mappings
IGMP snooping requires no communication between routers and switches it is based on the idea that the lan switch intercepts all IGMP related traffic and is able by itself to build the oilist for each multicast group.
So you don't need to enable cgmp if all your switches use igmp snooping that is the case with all modern switches
do sh igmp snooping to see if it is supported and enabled on the L2 switches
Hope to help