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Overview

Note: PIM has gained general acceptance among a large number of multicast-enabled networks. We recommend that you use PIM rather than DVMRP for applications that are not otherwise required to run DVMRP.

DVMRP is a dense-mode multicasting protocol and therefore uses a broadcast and prune mechanism. The protocol builds a source-rooted tree (SRT) in a similar way to PIM dense mode. DVMRP routers flood datagrams to all interfaces except the one that provides the shortest unicast route to the source. DVMRP uses pruning to prevent unnecessary sending of multicast messages through the SRT.

A DVMRP router sends prune messages to its neighbors if it discovers that:

The network to which a host is attached has no active members of the multicast group.
All neighbors, except the next-hop neighbor connected to the source, have pruned the source and the group.

When a neighbor receives a prune message from a DVMRP router, it removes that neighbor from its (S,G) pair table, which provides information to the multicast forwarding table.

When a host on a previously pruned branch attempts to join a multicast group, it sends an IGMP message to its first-hop router. The first-hop router then sends a graft message upstream.

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