Avaya Configuring IP Multicasting and Multimedia Services Manuel d'utilisateur télécharger pdf (Page 94)

Configuring IP Multicasting and Multimedia Services

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117355-D Rev 00

At startup, a DVMRP multicast router:

1. Initializes its routing table with information about all of its local networks

2. Learns the existence of its neighbors by sending a probe for all routes on each

of its multicast interfaces

3. Receives reports from its neighbors containing the routing information

(including route costs)

In Figure 4-1

, for example, router D becomes active and issues routing probes on

four multicasting interfaces. Router D receives reports from its multicasting

neighbors, routers B, C, and E.

Source Route Advertisements

A source network is any network containing hosts that can issue multicast

datagrams. DVMRP advertises shortest-path routes to multicasting source

networks. In this respect, DVMRP is the opposite of RIP, which advertises routes

to destination networks.

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

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

represents the sending router’s cost to reach the specified source 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 reexamines its

routing table to determine whether 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 and stores the route with the lower cost in its routing table.

A router will not send route reports on an interface until it knows (by means of

received probes or reports) that it has a neighboring multicast router on that

interface. It will continue to send probes periodically on an interface.

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Avaya Configuring IP Multicasting and Multimedia Services Manuel d'utilisateur Page 94