Advertising Routes

As its name suggests, DVMRP uses a distance-vector routing algorithm. Such algorithms require that each router periodically inform its neighbors of its routing table. DVMRP routers advertise routes by sending DVMRP report messages. For each network path, the receiving router picks the neighbor advertising the lowest cost and adds that entry to its routing table for future advertisement.

The cost, or metric, for this routing protocol is the hop count back to the source. The hop count for a network device is the number of routers on the route between the source and that network device.

Table 7 shows an example of the routing table for a DVMRP router.

Table 7: Sample Routing Table for a DVMRP Router

The DVMRP router maintains an (S,G) pair table that provides information to the multicast forwarding table. The (S,G) pair table is based on:

• Information from the DVMRP routing table
• Information learned from prune messages
• If IGMP and DVMRP are on the same interface, group information learned from IGMP

The (S,G) pair table includes a route from each subnetwork that contains a source to each multicast group of which that source is a member. These routes can be static or learned routes. Table 8 shows an example of the (S,G) pair table for DVMRP.

Table 8: Sample DVMRP (S,G) Pair Table

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