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Configuring IP Services

7-6

3. Receives reports from its neighbors containing the routing information

(including route costs)

In Figure 7-1, 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.

A router will not send out 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.

Calculating a Route Metric and Comparing Route Costs

Each interface — either a physical interface to a local network or a tunneled

interface to a remote multicasting router — is conﬁgured with a metric that

indicates the cost of the hop. A route metric is the sum of all the interface (hop)

metrics from a given route source to a given router. (Currently, mrouted restricts a

route to a total metric value of 31 or less.)

A router that receives multiple route reports for the same multicasting source

network compares the cost speciﬁed in each (based on the metric ﬁeld) and stores

information from the report with the lowest cost in its routing table.

In Figure 7-1, for example, Router D receives two reports for the network

connected to multicasting Router A, one from Router B and one from Router C.

Using the metrics contained in the route reports, Router D determines that the cost

of the tunneled route is greater than the cost of the route that uses direct physical

connections. Router D discards the route received from Router C and stores the

route received from Router B.

Router D then declares Router B to be the next-hop neighbor and Interface d1 to

be the next-hop interface. Once a next-hop neighbor has been declared for a route,

the route updates received from that neighbor for that route take precedence until

either the route times out or another router advertises a better metric for that route.

For information on using Site Manager to assign a cost metric to a DVMRP interface,

see the Metric parameter on page 7-22. For information on assigning a cost metric to a

tunnel, see the tunnel Metric parameter on page 7-25.

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Avaya Configuring IP Services Manual do Utilizador Página 288