Overview

DVMRP, which is used for routing multicasts within a single, autonomous system, is designed to be used as an interior gateway protocol (IGP) within a multicast domain. It is a distance-vector routing protocol that relies on IGMP functionality to provide connectionless datagram delivery to a group of hosts across a network.

DVMRP routes multicast traffic using a technique known as reverse path forwarding (RPF). When a router receives IP multicast packets, it first does an RPF check to determine if the packets are received on the correct interface. If so, the router forwards the packets out to the following:

• Local IGMP receivers for that group on interfaces for which the transmitting router is the designated forwarder
• Neighbor routers that have indicated their dependence on the transmitting router for forwarding multicast packets from that source (this is determined during DVMRP Route Exchange) and from which the transmitting router has not received any prune messages.

If not, the packets are discarded by the router. The transmitting router does not forward the packets back to the source.

If a router is attached to a set of VLANs that do not want to receive from a particular multicast group, the router can send a prune message back up the distribution tree to stop subsequent packets from traveling where there are no members. DVMRP periodically re-floods in order to reach any new hosts that want to receive from a particular group.

DVMRP routers dynamically discover their neighbors by sending neighbor probe messages periodically to an IP multicast group address that is reserved for all DVMRP routers.

Key features of DVMRP are the following:

• uses the well-known multicast IP address 224.0.0.4
• uses IGMP to exchange routing datagrams
• does not require an underlying Layer 3 routing protocol to provide a path to remote multicast destinations
• combines many of the features of the Routing Information Protocol (RIP) with the Truncated Reverse Path Broadcasting (TRPB) algorithm to route multicast packets between sources and receivers