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Customizing DVMRP

117355-C Rev 00

4-7

Shortest-Path Trees

Route information used by DVMRP is independent of any other routing

information used by the router -- for example, routes provided by RIP. The

purpose of this routing information is to create a shortest-path tree entry in the

routing table for the propagation of multicast datagrams.

The shortest-path tree entry indicates the interface that provides the shortest path

to the network that is the source of the multicast datagram.

In Figure 4-1

on page 4-3, for example, the routing table on router D includes an

entry describing the shortest-path route to the network connected to router A. The

entry indicates that Interface d1 provides the shortest path to that source network.

A shortest-path tree also indicates those interfaces that are on the shortest path to

that source network from a neighboring router.

Router E considers the network connecting it to router D to be on the shortest path

to the source network connected to router A. Router D has an interface (d3) that is

part of the shortest path from router E from the network connected to router A.

If neighboring routers have the same metric to a given source network, the router

with the lower IP address is responsible for propagating multicast traffic

originating from that source network onto the network or tunnel that is common to

these neighboring routers.

A network is considered a leaf network if it has no dependent downstream

neighbors for a source. In Figure 4-1

, the network connected to router E (net 5) is

a leaf network.

Tree Pruning

If no members are detected on a branch of the shortest path tree for a given group,

DVMRP prunes the branch.

The following conditions cause DVMRP to prune the shortest-path tree for a given

group:

• The router determines that the attached network on which the host is located

(or any other locally attached network) has no active member of the group.

• The router determines that all neighbors on all attached networks other than

the next-hop network to the source have pruned off the source and group.

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Avaya Configuring IP Multicasting and Multimedia Services Bedienungsanleitung Seite 65