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Configuring IP Multicasting

7-7

Advertising a Route

Periodically, each multicasting router issues full or partial routing information on

each DVMRP circuit, using DVMRP report messages. This routing information

represents the sending router’s cost to reach the speciﬁed network (the cost is the

sum of the hop metrics along the shortest path to the given source network).

Upon receiving a DVMRP report from another router, DVMRP re-examines its

routing table to determine if the shortest path information needs updating.

Speciﬁcally, DVMRP looks in the routing table for an entry describing a route to

the same source network. If one exists, DVMRP compares the cost of the two

routes. DVMRP stores the route with the lower cost in its routing table. (Other

received routing information is used in the construction of a shortest-path tree, as

described in “Creating a Shortest-Path Tree” on page 7-7.)

Creating a Shortest-Path Tree

Route information used by DVMRP is independent of any other routing

information used by the router — for example, routes provided by OSPF. 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 from a particular

source network to that router.

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

path from that source network to a neighboring router.

In Figure 7-1, for example, the routing table on Router D includes an entry

describing the shortest-path route from the network connected to Router A. The

route indicates that Interface d1 provides the shortest path to Router D from that

network. Router E considers the network between itself and Router D to be on the

shortest path from the network connected to router A. Router D has an interface

— d3 — that is part of the shortest path to Router E from the network connected

to Router A.

For information on using Site Manager to specify the interval at which DVMRP issues

routing information, see the Full Update Interval parameter on page 7-15 and the

Triggered Update Interval parameter on page 7-15.

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Avaya Configuring IP Services Manual de usuario Pagina 289