IGMP snooping

IGMP snooping is described in an informational IETF RFC but affects bridging operations, the purview of the IEEE.

A switch will, by default, flood multicast traffic to all the ports in a broadcast domain (or the VLAN equivalent).

Multicast can cause unnecessary load on host devices by requiring them to process packets they have not solicited.

IGMP snooping is designed to prevent hosts on a local network from receiving traffic for a multicast group they have not explicitly joined.

The next time there is a routine query from the router the switch will forward the reply from the remaining host.