IGMP snooping is a feature used to allow a Layer 2 switch to constrain multicast traffic within a VLAN to be sent only to those hosts that have requested it. Routers use Protocol Independent Multicast (PIM) to determine where to route multicast traffic. Layer 2 switches use IGMP snooping.
A switch achieves this by listening to IGMP traffic between the router and hosts:
- When the host sends a membership report for a multicast group then the switch adds an entry in the CAM table for the interface that is connected to the host.
- When the host sends a leave group for a multicast group then the switch removes an entry in the CAM table for the interface that is connected to the host.
Based on this information, a switch can send multicast traffic only to those hosts that requested it.