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6
Configuring Routing
DVMRP (Distance Vector Multicast Routing
Protocol)
DVMRP Description
DVMRP is a distance vector protocol that calculates a source-rooted multicast
distribution tree and provides routing of IP multicast datagrams over an IP
internetwork. DVMRP uses a distance vector (Bellman-Ford) routing
protocol to maintain topological knowledge. DVMRP uses this information to
implement a multicast forwarding algorithm, Reverse Path Flooding (RPF).
RPF forwards a multicast datagram to members of the destination group along
a shortest (reverse) path tree that is rooted at the subnet on which the
datagram originates. Truncated Reverse Path Broadcasting (TRPB) uses the
IGMP-collected group membership state to avoid forwarding on leaf
networks that do not contain group members.
TRPB calculates a distribution tree across all multicast routers and only saves
packet transmissions on the leaf networks that do not contain group members.
Reverse Path Multicast (RPM) allows the leaf routers to prune the distribution
tree to the minimum multicast distribution tree. RPM minimizes packet
transmissions by not forwarding datagrams along branches that do not lead to
any group members.
Multicast capabilities are not always present in current Internet-based
networks. Multicast packets must sometimes pass through a router that does
not support IP multicasting in order to reach their destination. This is allowed
because DVMRP defines a virtual tunnel interface between two multicast-
capable routers that may be connected by multiple non-multicast capable IP
hops.
DVMRP encapsulates IP multicast packets for transmission through tunnels
so that they look like normal unicast datagrams to intervening routers and
subnets. DVMRP adds the encapsulation when a packet enters a tunnel and
removes it when the packet exits from a tunnel. The packets are encapsulated
using the IP-in-IP protocol (IP protocol number 4). This tunneling mechanism
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Voyager Reference Guide
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