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Multicast Routing
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Configuring DVMRP
The Distance-Vector Multicast Routing Protocol (DVMRP) behaves somewhat
similarly to RIP. A router supporting DVMRP periodically floods its attached networks
to pass information about supported multicast services along to new routers and
hosts. Routers that receive a DVMRP packet send a copy out to all paths (except the
path back to the origin). These routers then send a prune message back to the
source to stop a data stream if the router is attached to a LAN which does not want
to receive traffic from a particular multicast group. However, if a host attached to this
router issues an IGMP message indicating that it wants to subscribe to the
concerned multicast service, this router will use DVMRP to build up a source-rooted
multicast delivery tree that allows it to prevent looping and determine the shortest
path to the source of this multicast traffic.
When this router receives the multicast message, it checks its unicast routing table
to locate the port that provides the shortest path back to the source. If that path
passes through the same port on which the multicast message was received, then
this router records path information for the concerned multicast group in its routing
table and forwards the multicast message on to adjacent routers, except for the port
through which the message arrived. This process eliminates potential loops from the
tree and ensures that the shortest path (in terms of hop count) is always used.
Configuring Global DVMRP Settings
DVMRP is used to route multicast traffic to nodes which have requested a specific
multicast service via IGMP. This router uses Reverse Path Forwarding (RPF) to
build a shortest-path delivery tree that begins at the source and spreads out to reach
group members through the network. RPF uses three different techniques to
dynamically reconfigure the multicast spanning tree: broadcasting, pruning, and
grafting.
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