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 is interested in receiving the data. Such scenarios can lead to loss of throughput for switches that receive a high rate of multicast data traffic.
IGMP snooping is a mechanism by which a Layer 2 device can effectively address the issue of inefficient multicast forwarding to VLAN port members. Snooping involves "learning" forwarding states for multicast data traffic on VLAN port members from the IGMP control (join and leave) packets received on them. The Layer 2 device also provides for a way to configure forwarding states statically through the CLI.