Topology Changes with Feasible Successor

In Figure 8-3, router 4 wants to reach network N. Router 4 has two choices: either through router 2 or through router 1. Each interface has a pretend metric (45 and 55, respectively) as an example.

Figure 8-3. Convergence Due to Feasible Successor

Figure 8-3. Convergence Due to Feasible Successor

The metric from router 3 to its destination is 40, and this metric is less than router 4's current metric (45) via router 2 to reach network N. Therefore, the feasibility condition is satisfied. In this case, router 2 is the successor for router 4 to reach network N, and router 3 is the feasible successor.

Now, assume that the link between router 4 and router 2 fails. Router 4 will not enter an active state. In the same manner, assume that the link between router 2 and router 1 fails, which means that router 2 will enter the active state from destination N. Router 2 will send queries to all its neighbors. Router 4 determines that it has a feasible successor, and replies immediately with a metric of 55.

Now, router 4 will change its successor from router 2 to router 3; the router 4 for its destination N is in a passive state. Router 2 will receive the reply, and can transition from active to passive state because it has received a reply for its queries from router 4. In this case, router 4 was the only neighbor, so queries from router 2 have been answered by router 4. The other neighbor, router 1, is down. Note that router 1 and router 3 were not involved in the recomputation because they were not affected by the change.

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