D-Link DES-3326S Multicast Routing Protocols, Distance Vector Multicast Routing Protocol Dvmrp

DES-3326S Layer 3 Fast Ethernet Switch User’s Guide

Multicast Routing Protocols

This section contains an overview of two multicast routing protocols – Distance Vector Multicast Routing Protocol (DVMRP), and Protocol Independent Multicast-Dense Mode

(PIM-DM). The most commonly used routing protocol (not a multicast routing protocol), the Routing Information Protocol, is discussed in a later section.

Distance Vector Multicast Routing Protocol (DVMRP)

The Distance Vector Multicast Routing Protocol (DVMRP) was derived from the Routing Information Protocol (RIP) with the introduction of multicast delivery trees constructed from information about the ‘distance’ from the local router back toward the multicast source. DVMRP uses an RPM algorithm to construct its multicast delivery trees.

The first multicast packet received by a multicast router using DVMRP is flooded to all interfaces except the one on which the packet was received. Subsequent prune messages are used to prune branches of the delivery tree that are either not on the shortest path back to the multicast source, or that have no active multicast group members. A ‘graft’ message is added that allows a previously pruned branch of the multicast delivery tree to be reactivated. This allows for lower latency when a leaf router adds a new member to a multicast membership group. Graft messages are forwarded one hop (one router) back at a time toward a multicast source until they reach a router that is on an active branch of the multicast delivery tree.

If there is more than one multicast router on a network, the one that has the shortest path back to the multicast source is elected to forward multicast packets from that source. All

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