Configuring IP Multicast
Describing the IGMP snooping process

The default behavior for a Layer 2 switch is to forward multicast traffic to every port in the VLAN on which the traffic was received. Therefore, a switch between a requesting host and a multicast router will forward a multicast flow intended for a single host out all switch ports on the same VLAN as the receiving host. IGMP snooping is an IP multicast constraining mechanism for switches. It examines IGMP frames so that multicast traffic is not forwarded out all VLAN ports but only those over which hosts sent IGMP message toward the router.

IGMP snooping runs on a Layer 2 switch. The switch snoops the content of the IGMP join and leave messages sent between the hosts and the router. When the switch sees an IGMP report from a host to join a particular multicast group, the switch creates a CAM table entry associating the port number where that message was seen to the Layer 2 multicast address for the group that the host joined. When the frames of the multicast flow arrive at the switch with the destination multicast MAC address, they are forwarded down only those ports where the IGMP messages were snooped, and associated CAM table entries were created. When the switch snoops the IGMP leave group message from a host, the switch removes the table entry.