
Customizing DVMRP
114064 Rev. B 4-5
In Figure 4-1, 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.
Routing Table and Forwarding Table
Table 4-1 summarizes the principal items in a routing table entry.
Note that the source subnet and the previous-hop router in the DVMRP routing
table are the opposite of the destination subnet and next-hop router in a RIP
routing table.
Using this information, the router can
1. Receive a multicast datagram and determine if the datagram has arrived on the
interface that is on the shortest path to the source network.
2. Drop the datagram if it has not arrived on the shortest-path interface.
3. Flood the multicast datagram to all active DVMRP neighbors.
Table 4-1. Routing Table Entry Items
Item Description
Source subnet address and mask Address and mask of a subnetwork
containing a host sourcing multicast
datagrams
From-gateway Previous-hop router leading back to the
source subnet
TTL Number of seconds before this entry is
removed from the routing table
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