Preconvergence Routing Table Contents

If packets sent by Router C to Server suddenly become undeliverable, it is likely that an error occurred somewhere in the network. This could have been caused by a seemingly infinite number of different, specific failures. Some of the more common suspects include the following:

• The server has failed completely (due to either a hardware, software, or electrical failure).

• The LAN connection to the server has failed.

The Mechanics of Routing Protocols

• Router D has experienced a total failure.

• Router D's serial interface port to router C has failed.

• The transmission facility between Gateway Routers C and D has failed.

• Router C's serial interface port to Router D has failed.

Obviously, the new network topology can't be determined until the exact location of the failure has been identified. Similarly, the routers cannot attempt to route around the problem until the failure location has been isolated. If either of the first two scenarios occurred, server would be completely unavailable to all the users of the internetwork, regardless of any route redundancy that may have been built into the network.

Similarly, if router D had failed completely, all the LAN-attached resources at that location would be isolated from the rest of the network. If the failure was either a partial failure of that router, or elsewhere in the network, however, there might still be a way to reach Server Finding a new route to requires the network's routers to recognize and agree on which piece of the network failed. In effect, subtracting this component from the network changes the network's topology.

To continue with the example, assume that Router D's serial interface port to router C has failed. This renders the link between C and D unusable. Figure 7-5 illustrates the new network topology.

Figure 7-5: The link between Routers C and D is unusable.

Figure 7-5: The link between Routers C and D is unusable.

Routers using a dynamic routing protocol would quickly determine that Server was unreachable through their current, preferred route. Individually, none of the routers could determine where the actual failure occurred, nor could they determine whether any viable alternative routes still existed. By sharing information with each other, however, a new composite picture of the network can be developed.

Note For the purposes of this chapter, this example uses an intentionally generic method of convergence.

The Mechanics of Routing Protocols

More specific details about each routing protocol's convergence characteristics are presented in Part III.

The routing protocol used in this internetwork is relatively simple. It limits each router to exchanging routing information with its immediate neighbors, although it supports the recording of multiple routes per destination. Table 7-6 summarizes the pairs of immediately adjacent routers illustrated in Figure 7-5.

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