JUNOS Enterprise Routing

Multicast Protocols

This section describes the operation of group management and multicast routing protocols. We will focus on PIM sparse mode because it’s the predominate form of multicast routing protocol in modern IP multicast networks. Simply stated, group management protocols are run by hosts to inform local routers of a host’s interest, or lack thereof, in a particular multicast group. Multicast routing protocols are run only on routers and are concerned with RPF checks and the establishment and maintenance of (*,G) and (S,G) forwarding state.

Group Management Protocols

IGMP performs multicast group management and is run on hosts and on routers that attach to host segments. IGMP versions 1, 2, and 3 are currently defined in RFCs 1112, 2236, and 3376, respectively. The basic mechanics of IGMP operation center on hosts generating report messages to inform attached routers what groups the host is interested in, and to inform routers generating query messages to determine whether any active listeners still remain for a particular group.

There are three versions of IGMP—Juniper routers default to version 2, but you can configure them for version 1 or version 3 as needed. Although the various versions of IGMP are backward-compatible, this compatibility is achieved at the cost of having to drop back to the lowest common denominator. For example, if one host is running IGMPv1, any router attached to the LAN running IGMPv2 drops back to IGMPv1 operation, effectively eliminating ...