"Multicast in the Layer 2 Switching Environment
The default behavior for a Layer 2 switch would be to forward all multicast traffic to every port that belongs to the destination LAN on the switch. This would defeat the purpose of the switch, which is to limit traffic to the ports that need to receive the data. There are two methods to deal with multicast in a Layer 2 switching environment efficiently-Cisco Group Management Protocol (CGMP) and IGMP Snooping. CGMP is a Cisco developed protocol that allows Catalyst® Switches to take advantage of IGMP information on Cisco routers to make Layer 2 forwarding decisions. CGMP has to be configured both on the multicast routers and the Layer 2 switches. The net result is that with CGMP, IP multicast traffic is delivered only to those Catalyst Switch ports that are interested in the traffic. All other ports that have not explicitly requested the traffic will not receive it. IGMP Snooping requires the LAN switch to examine, or "snoop" some Layer 3 information in the IGMP packets sent between the hosts and the router. When the switch hears the IGMP Host Report from a host for a particular multicast group, the switch adds the host port number to the associated multicast table entry. When the switch hears the IGMP Leave Group message from a host, it removes the host port from the table entry."
IGMP/CGMP/IGMP snooping are all protocols for the layer-2 switch to figure out which hosts on which ports are members of what multicast groups so the switch doesn't have to forward all multicast traffic to all ports.
PIM is used only on routers. The routers in a network use PIM to determine which of them should be sent multicast traffic. They do this because it is possible not all routers have hosts hanging off of them on a subnet that are in the multicast group and therefore don't need to be sent any of that particular multicast traffic.
-HTH
