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Understanding IGMP

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.

1. To join a multicast group, G, a host conveys its membership information through IGMP.
2. The router then forwards data packets addressed to a multicast group to only those interfaces on which explicit join messages have been received.
3. A designated router (DR) sends periodic join and prune messages toward a group-specific rendezvous point (RP) for each group for which it has active members. One or more routers are automatically or statically designated as the RP, and all routers must explicitly join through the RP.
4. Each router along the path toward the RP builds a wild card (any-source) state for the group and sends join and prune messages toward the RP.

   The term route entry is used to refer to the state maintained in a router to represent the distribution tree.

   A route entry can include such fields as:

   • source address
   • group address
   • incoming interface from which packets are accepted
   • list of outgoing interfaces to which packets are sent
   • timers
   • flag bits

   The wild card route entry's incoming interface points toward the RP.

   The outgoing interfaces point to the neighboring downstream routers that have sent join and prune messages toward the RP as well as the directly connected hosts that have requested membership to group G.

5. This state creates a shared, RP-centered, distribution tree that reaches all group 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.