DUAL Operation

Typical distance vector protocols, such as RIP, use the distance (metric—in most cases, the hop count) to a destination network to determine the best path and save the vector (next hop) for only the best path. If the best path becomes unusable, the router waits until the next set of updates from each of its neighbors to find a new path (or rediscover an old path that was previously discarded).

Waiting for periodic updates to discover alternate paths to a destination slows convergence time dramatically.

For example, if the network in Figure C-1 is running RIP, Router B will choose the path to by examining the hop count through each available path. Because the path through Router C is three hops, and the path through Router A is two hops, Router B will choose the path through Router A and discard the alternate path it learned through Router C.

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