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.This is simultaneously a source of strength and vulnerability.Under normal operatingconditions, this independent and distributed intelligence is a source of tremendous advantage.Duringchanges in the network's topology, the process of converging on a new consensus of the network's shapemay actually introduce instability and routing problems.Accommodating Topological ChangesUnfortunately, the independent nature of routers can also be a source of vulnerability whenever a changeoccurs in the network's topology.Such changes, by their very nature, change a network's topology.Figure 7-4 illustrates how a change in the network is, in fact, a change in its topology.Figure 7-4: A four-gateway internetwork.http://wwwin.cisco.com/cpress/cc/td/cpress/fund/iprf/ip2907.htm (11 of 19) [02/02/2001 11.37.52]The Mechanics of Routing ProtocolsFigure 7-4 features another fairly simple, four-node internetwork with some route redundancy.Table 7-5summarizes the routing tables of the four routers.For the sake of this example, consider this table to bepreconvergence routing table information.Table 7-5:Router Destination Next Hop Number of Hops to DestinationA 172.16.0 B 1A 192.168.125.0 C 1A 192.168.253.0 B or C 2B 10.0 A 1B 192.168.125.0 A or D 2B 192.168.253.0 D 1C 10.0 A 1C 172.16.0 A or D 2C 192.168.253.0 D 1D 10.0 B or C 2D 172.16.0 B 1D 192.168.125.0 C 1Preconvergence Routing Table ContentsIf packets sent by Router C to Server 192.168.253.2 suddenly become undeliverable, it is likely thatan error occurred somewhere in the network.This could have been caused by a seemingly infinitenumber 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.http://wwwin.cisco.com/cpress/cc/td/cpress/fund/iprf/ip2907.htm (12 of 19) [02/02/2001 11.37.52]The Mechanics of Routing ProtocolsRouter 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 beenidentified.Similarly, the routers cannot attempt to route around the problem until the failure location hasbeen isolated.If either of the first two scenarios occurred, server 192.168.253.2 would becompletely unavailable to all the users of the internetwork, regardless of any route redundancy that mayhave been built into the network.Similarly, if router D had failed completely, all the LAN-attached resources at that location would beisolated from the rest of the network.If the failure was either a partial failure of that router, or elsewherein the network, however, there might still be a way to reach Server 192.168.253.2.Finding a newroute to 192.168.253.2 requires the network's routers to recognize and agree on which piece of thenetwork 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.Thisrenders 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.Routers using a dynamic routing protocol would quickly determine that Server 192.168.253.2 wasunreachable through their current, preferred route.Individually, none of the routers could determinewhere the actual failure occurred, nor could they determine whether any viable alternative routes stillexisted.By sharing information with each other, however, a new composite picture of the network can bedeveloped.Note For the purposes of this chapter, this example uses an intentionally generic method of convergence.http://wwwin.cisco.com/cpress/cc/td/cpress/fund/iprf/ip2907.htm (13 of 19) [02/02/2001 11.37.52]The Mechanics of Routing ProtocolsMore 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 exchangingrouting information with its immediate neighbors, although it supports the recording of multiple routesper destination.Table 7-6 summarizes the pairs of immediately adjacent routers illustrated in Figure 7-5.Table 7-6: Routers that Share Routing Information with Immediate NeighborsRouter A B C DA --- Yes Yes NoB Yes --- No YesC Yes No --- YesD No Yes Yes ---The entries in Table 7-6 that contain the word Yes indicate a physically adjacent pair of routers thatwould exchange routing information.The entries that contain a dash denote the same router: A routercannot be adjacent to itself.Finally, those entries that contain the word No indicate nonadjacent routersthat cannot directly exchange routing information.Such routers must rely on their adjacent neighbors forupdates about destinations on nonadjacent routers.From this table, it is apparent that because they are not directly connected to each other, Routers A and Dmust rely on Routers B and C for information about each other's destinations.Similarly, Routers B and Cmust rely on Routers A and D for information about each other's destinations.Figure 7-6 shows this sharing of routing information between immediate neighbors.Figure 7-6: Immediate neighbors sharing routing data.http://wwwin.cisco.com/cpress/cc/td/cpress/fund/iprf/ip2907.htm (14 of 19) [02/02/2001 11.37.52]The Mechanics of Routing ProtocolsThe important implication in this scenario is that, because not every router is immediately adjacent toevery other router, more than one routing update may be required to fully propagate new routinginformation that accommodates the failed link.Therefore, accommodating topological change is aniterative and communal process.For the sake of simplicity, assume that convergence occurs within two routing table updates in thisexample.During the first iteration, the routers are starting to converge on a new understanding of theirtopology.Routers C and D, because of the unusable link between them, cannot exchange routinginformation.Consequently, they invalidate this route and all destinations that use it [ Pobierz całość w formacie PDF ]