Understanding IGMP
The Internet Group Management Protocol (IGMP) manages the membership of hosts and routers in multicast groups. IP hosts use IGMP to report their multicast group memberships to any immediately neighboring multicast routers. Multicast routers use IGMP to learn, for each of their attached physical networks, which groups have members.
IGMP is also used as the transport for several related multicast protocols (for example, Distance Vector Multicast Routing Protocol [DVMRP] and Protocol Independent Multicast version 1 [PIMv1]).
IGMP is an integral part of IP and must be enabled on all routers and hosts that need to receive IP multicast traffic.
For each attached network, a multicast router can be either a querier or a nonquerier. The querier router periodically sends general query messages to solicit group membership information. Hosts on the network that are members of a multicast group send report messages. When a host leaves a group, it sends a leave group message.
IGMP version 3 (IGMPv3) supports inclusion and exclusion lists. Inclusion lists enable you to specify which sources can send to a multicast group. This type of multicast group is called a source-specific multicast (SSM) group, and its multicast address is 232/8.
IGMPv3 provides support for source filtering. For example, a router can specify particular routers from which it accepts or rejects traffic. With IGMPv3, a multicast router can learn which sources are of interest to neighboring routers.
Exclusion mode works the opposite of an inclusion list. It allows any source but the ones listed to send to the SSM group.
IGMPv3 interoperates with versions 1 and 2 of the protocol. However, to remain compatible with older IGMP hosts and routers, IGMPv3 routers must also implement versions 1 and 2 of the protocol. IGMPv3 supports the following membership-report record types: mode is allowed, allow new sources, and block old sources.
For information about supported standards for IGMP, see Supported IP Multicast Protocol Standards.
