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Distance Vector Protocol Problems and Solutions
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doesn t contain 19216830 and RouterC s routing table doesn t contain 10000 Note, however, that RouterB has converged, but RouterA and RouterC still need additional routes After their periodic timers expire, the routers again generate local routing broadcast updates on each of their interfaces Again, they broadcast their entire routing tables on these interfaces Figure 15-3 shows the network after these routers process these new updates The routers in this network go through the same process again when receiving the updates Notice that RouterA s routing table now contains 19216830, with a hop count of 2, while RouterC s routing table contains 10000, with a hop count of 2 Both of these routers learned these networks via RouterB And since these networks have a hop count of 1 on RouterB, when the edge routers receive the routing table from RouterB, they increment the hop count to 2 for these network numbers Given the routing tables shown in Figure 15-3, the routers have fully converged The problem is, however, that convergence took place only after two updates The first update took place as soon as the interface was active, and the second update took place 60 seconds later So in this example, it took more than 60 seconds for convergence to occur You can imagine that if you have a few hundred routers in your network, it might take many minutes before your network converges and each router knows about all of the destinations that are reachable
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FIGURE 15-3
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Convergence example after second routing update
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15: Routers and Routing
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FIGURE 15-4
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RouterA s E0 interface has failed
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Let s use the same network, but assume that RouterA s E0 interface has failed and that RouterA has lost its connection to network 10000, as shown in Figure 15-4 As you can see in this example, RouterA s routing table lists the network as unreachable Unfortunately, RouterA cannot tell the rest of the network concerning the downed route until its periodic timer expires After the timer expires, RouterA advertises its routing table to RouterB, which is shown in Figure 15-5 After RouterB receives its update, it has converged However, RouterC is still lacking this information about the updated topology and must wait for RouterB s periodic timer to expire in order to receive RouterB s updated routing table After RouterB s periodic timer has expired, it shares its routing table with RouterC, as is shown in Figure 15-6 Up to this point, RouterC assumed that it had the most upto-date routing information and would still send packets to 10000, since the routing table indicated that 10000 was reachable via RouterB However, after receiving the routing update from RouterB, RouterC updates its routing table and knows that 10000 is not reachable; it will now drop any packets being sent to 10000
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Convergence occurs when all routers understand the current topology of the network You should be able to gure
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out whether a distance vector protocol has or hasn t converged by examining the routing tables on routers
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Distance Vector Protocol Problems and Solutions
FIGURE 15-5
RouterB receives the updated information
Now all three routers have converged Here are the three things that affected convergence in this example: the time it took for RouterA to discover that E0 failed (a few seconds); the periodic timer on RouterA to advertise this to RouterB (up to 60 seconds); and the periodic timer on RouterB to advertise this to RouterC (up to 60 seconds) Given these three items, it could take more than 2 minutes to converge As you can see from the past two examples, convergence with distance vector protocols is a slow process
FIGURE 15-6
RouterC receives the updated information
15: Routers and Routing
Solution:Triggered Updates
Now that you understand some of the problems associated with convergence in distance vector protocols, consider one possible solution Given the three things listed in the preceding paragraph that affected convergence with the unreachable network (10000), the two things that slowed down convergence were periodic timers You can actually use two solutions to speed convergence: change the periodic timer interval and/or use triggered updates The first solution is to change the periodic timer interval For instance, in our example the timer was set to 60 seconds To speed up convergence, you might want to set the interval to 10 seconds In this example, then, convergence would take only about 20 seconds However, in today s networks, even waiting this amount of time creates network disruptions Also, by setting the timer to 10 seconds, you are creating six times the amount of routing broadcast traffic, which is not very efficient The second solution is to implement triggered updates Triggered updates complement periodic updates The distance vector routing protocol would still generate periodic updates; however, whenever a change took place, the router would immediately generate an update without waiting for the periodic timer to expire This can decrease convergence times, but it also creates a problem If you had a flapping route, an update would be triggered each time the route changed state, which would create a lot of unnecessary broadcast traffic in your network and could cause a broadcast storm
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