
Customizing DVMRP
117355-A Rev. A 4-5
How DVMRP Chooses a Route
Each DVMRP interface is configured with a metric that indicates the cost of the
hop. A router that receives multiple route reports for the same multicasting source
network
• Compares the cost specified in each (based on the metric field)
• Stores information from the report with the lowest cost in its routing table
A route metric is the sum of all the interface (hop) metrics from a given route
source to a given router.
In F
igure 4-1 on page 4-3, 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. After 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 until another router advertises a better metric for that
route.
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