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| 7.3 |  | Routing Multicast Traffic |
| 7.3.6 |  | Distance Vector Multicast Routing Protocol (DVMRP) |

Distance Vector Multicast Routing Protocol (DVMRP) was the first true multicast routing protocol to see widespread use. DVMRP is similar in many ways to the Routing Information Protocol (RIP) with some variations added to support multicast. Described in RFC 1075, DVMRP is widely used on the Internet multicast backbone (MBONE).

DVMRP uses reverse-path flooding. When a router receives a packet, it floods the packet out of all paths except the one that leads back to the packet source. This technique allows a data stream to reach all LANs. If a router is attached to a set of LANs that do not want to receive 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 will periodically flood packets in order to reach any new hosts that want to receive traffic from a particular multicast group. There is a direct relationship between the time it takes for a new receiver to get the data stream and the frequency of flooding.

DVMRP implements its own unicast routing protocol in order to determine which interface leads back to the source of the data stream. This unicast routing protocol is similar to RIP and is based purely on hop counts. As a result, the path that the multicast traffic follows may not be the same as the path that the unicast traffic follows.

> Note: Cisco routers run PIM. Cisco routers know enough about DVMRP to successfully forward multicast packets to and receive packets from a DVMRP neighbor. Cisco routers can also propagate DVMRP routes into and through a PIM cloud. However, only the PIM protocol uses this information. Cisco routers do not implement DVMRP to forward multicast packets.