Note

Convergence and convergence time

The process of bringing all route tables to a state of consistency is called convergence. The time it takes to share information across an internetwork and for all routers to calculate best paths is the convergence time.

Figure 4.2 shows an internetwork that was converged, but now a topology change has occurred. The link between the two left-most routers has failed; both routers, being directly connected, know about the failure from the data link protocol and proceed to inform their neighbors of the unavailable link. The neighbors update their route tables accordingly and inform their neighbors, and the process continues until all routers know about the change.

Figure 4.2. Reconvergence after a topology change takes time. While the internetwork is in an unconverged state, routers are susceptible to bad routing information.

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Figure 4.2. Reconvergence after a topology change takes time. While the internetwork is in an unconverged state, routers are susceptible to bad routing information.

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Notice that at time t2 the three left-most routers know about the topology change but the three right-most routers have not yet heard the news. Those three have old information and will continue to switch packets accordingly. It is during this intermediate time, when the internetwork is in an unconverged state, that routing errors may occur. Therefore convergence time is an important factor in any routing protocol. The faster a network can reconverge after a topology change, the better.

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