3.2 Convergence
3.2.2 Accommodating topological changes
Unfortunately, the independent nature of routers can also be a source of vulnerability whenever a change occurs in the network's topology. Such changes, by their very nature, change a network's topology. Figure illustrates how a change in the network is, in fact, a change in its topology.

Figure features another fairly simple, four-node internetwork with some route redundancy. Figure summarizes the routing tables of the four routers. For the sake of this example, consider this table to be preconvergence routing table information.

Preconvergence Routing Table Contents

If packets sent by Router C to Server 192.168.253.2 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.
  • 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 192.168.253.2 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 192.168.253.2. Finding a new route to 192.168.253.2 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 illustrates the new network topology.

Routers using a dynamic routing protocol would quickly determine that Server 192.168.253.2 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. 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. Figure summarizes the pairs of immediately adjacent routers illustrated in Figure .

The entries in Figure that contain the word Yes indicate a physically adjacent pair of routers that would exchange routing information. The entries that contain a dash (gray) denote the same router: A router cannot be adjacent to itself. Finally, those entries that contain the word No indicate nonadjacent routers that cannot directly exchange routing information. Such routers must rely on their adjacent neighbors for updates about destinations on nonadjacent routers.

From this table, it is apparent that because they are not directly connected to each other, Routers A and D must rely on Routers B and C for information about each other's destinations. Similarly, Routers B and C must rely on Routers A and D for information about each other's destinations.

Figure shows this sharing of routing information between immediate neighbors.

The important implication in this scenario is that, because not every router is immediately adjacent to every other router, more than one routing update may be required to fully propagate new routing information that accommodates the failed link. Therefore, accommodating topological change is an iterative and communal process.

For the sake of simplicity, assume that convergence occurs within two routing table updates in this example. During the first iteration, the routers are starting to converge on a new understanding of their topology. Routers C and D, because of the unusable link between them, cannot exchange routing information. Consequently, they invalidate this route and all destinations that use it. Figure summarizes the contents of the four routers' routing tables during the convergence process. Note that the contents of some routing tables may reflect the mistaken belief that the link between Routers C and D is still valid.

In Figure , Routers C and D have invalidated the route between them. Routers A and B, however, still believe that their routes through this link are viable. They must await a routing update from either Router C and/or D before they can recognize the change in the internetwork's topology.

Figure contains the contents of the four routers' routing tables after they have converged on a new topology. Remember that this is an intentionally generic depiction of the convergence process; it is not indicative of any particular routing protocol's mechanics.

As evident in Figure , all the routers in the internetwork eventually agree that the link between C and D is unusable, but that destinations in each autonomous system are still reachable via an alternative route.