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.