A Layer 2 switch forwards all multicast control packets and data received on all the member ports of a VLAN interface. This simple approach is not bandwidth efficient, because only a subset of member ports may be connected to devices that want to receive these multicast packets. In a worst-case scenario, the data is forwarded to all port members of a VLAN even if only one VLAN member wants to receive the data. Such a scenario can lead to loss of throughput for switches that receive a high rate of multicast data traffic.
IGMP helps save bandwidth and throughput by forwarding traffic only to interested receivers. IGMP snooping provides the specification for forwarding IPv4 data traffic. For more information, see IGMP Traffic Snooping.
With PIM snooping, switches learn which multicast router ports need to receive multicast traffic. For more information, see IPv4 PIM-SM Traffic Snooping.