IGMP (Internet Group Management Protocol) allows hosts and routers to work together to optimize forwarding of multicast traffic on a network.
Without IGMP, all multicast packets must be forwarded to all network segments.
With IGMP, multicast traffic is only forwarded to network segments, which connect interested hosts.
An IGMP snooping switch performs many of the functions of an IGMP router.
Periodically, routers and IGMP snooping switches in active mode send an IGMP Query on each attached network. (The query interval is generally around 1-2 minutes.)