IGMP Basics

Basic IGMP operation involves the following two devices:

IGMP host (client)—Device that issues messages to join or leave a multicast group. This device also responds to queries from the multicast router. A set-top box is an example of an IGMP host.
IGMP router (multicast router)—Device that responds to the join and leave messages to determine whether or not to forward multicast groups from an interface. Periodic queries assist the router in recovering from any error conditions and verifying requests. The IGMP router receives multicast groups through the use of a multicast protocol, such as Protocol Independent Multicast (PIM), or through static flooding. An IGMP router is the termination point for any IGMP messages and therefore does not send any IGMP information to its upstream neighbors.

The IGMP protocol provides the following three basic functions for IP multicast networks:

Join messages—Messages that indicate an IGMP host wants to receive information from (that is, become a member of) a multicast group.
Leave messages—Messages that indicate an IGMP host no longer wants to receive information from a multicast group.
Query messages—Messages from an IGMP router requesting information from a host. For example, if a set-top box is unplugged without first issuing a leave message, the IGMP router may query the host to determine what multicast groups the host belongs to.