IGMP Commands 87345IGMP CommandsInternet Group Management Protocol (IGMP) is the multicast groupmembership discovery protocol used for IPv4 multicast groups. Threeversions of IGMP exist. Versions one and two are widely deployed. SinceIGMP is used between end systems (often desktops) and the multicast router,the version of IGMP required depends on the end-user operating systembeing supported. Any implementation of IGMP must support all earlierversions.The following list describes the basic operation of IGMP, common to allversions. A multicast router can act as both an IGMP host and an IGMProuter and as a result can respond to its own IGMP messages. ThePowerConnect implementation of IGMPv3 supports the multicast routerportion of the protocol (that is, not the host portion). It is backwardcompatible with IGMPv1 and IGMPv2.• One router periodically broadcasts IGMP Query messages onto thenetwork.• Hosts respond to the Query messages by sending IGMP Report messagesindicating their group memberships.• All routers receive the Report messages and note the memberships of hostson the network.• If a router does not receive a Report message for a particular group for aperiod of time, the router assumes there are no more members of thegroup on the network.All IGMP messages are raw IP data grams and are sent to multicast groupaddresses, with a time to leave (TTL) of 1. Since raw IP does not providereliable transport, some messages are sent multiple times to aid reliability.IGMPv3 is a major revision of the protocol and provides improved groupmembership latency. When a host joins a new multicast group on aninterface, it immediately sends an unsolicited IGMP Report message for thatgroup.
