qr code c# sample 15: Routers and Routing in Objective-C

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15: Routers and Routing
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Distance Vector Protocol Problems and Solutions
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The remainder of this chapter focuses on the problems that pertain to distance vector routing protocols: they converge slowly and they are prone to routing (layer 3) loops The next few sections cover these problems, as well as present solutions implemented by distance vector protocols to solve these problems
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Problem: Convergence
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The term convergence, in routing parlance, refers to the time it takes for all of the routers to understand the current topology of the network Link state protocols tend to converge very quickly, while distance vector protocols tend to converge slowly
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Convergence Example
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To understand the issue that distance vector protocols have with convergence, let s look at an example The network is shown in Figure 15-1 In this example, assume that the periodic timer for the distance vector protocol is set to 60 seconds Also assume that the distance vector protocol is using hop count as a metric and no special features are implemented in this example to solve convergence or routing loop problems
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FIGURE 15-1
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Convergence example after routers are turned on
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Distance Vector Protocol Problems and Solutions
This example has three routers, RouterA, RouterB, and RouterC, where these routers were just turned on As you can see from the routers routing tables, the only routes these routers initially know about are their directly connected routes, which they learn by examining the status of their interfaces, making sure that they are up and up; they then take the network numbers of these interfaces (learned from the configured IP address and subnet mask) and put this information in their routing tables Currently, each router contains two routes in its routing table Also notice the metric: these routes have a hop count of 0, since they are directly connected Now that their interfaces are active and the routers have an initial routing table, they ll send out their first routing broadcast on these interfaces (they don t wait for their periodic timer in this instance, since the interfaces just went active) This broadcast contains the entries that they have in their routing tables Assume that all routers are synchronized when they advertise their routing broadcasts, even though this would be highly unlikely in a production environment This list shows which routers are advertising which routes on their active interfaces:
RouterA RouterB RouterC
Networks 10000 and 19216810 Networks 19216810 and 19216820 Networks 19216820 and 19216830
After this first exchange of routing tables, each router will process its neighbor s received update and incorporate these changes, if necessary Figure 15-2 displays the contents of the routing tables on the routers after this first exchange
FIGURE 15-2
Convergence example after first routing update
15: Routers and Routing
Let s break this process down one router at a time, starting with RouterA: 1 Receives networks 19216810 and 19216820 from RouterB and increments the metric by one hop for each route 2 Compares the advertised routes from RouterB to what it has in its routing table 3 Adds 19216820 because it is not in the routing table 4 Ignores 19216810 from RouterB because RouterB has a hop count of 1, while the current routing table entry in the routing table has a hop count of 0 Let s look at RouterC next: 1 Receives networks 19216810 and 19216820 from RouterB and increments the metric by one hop 2 Compares the advertised routes from RouterB to what it has in its routing table 3 Adds 19216810 because it is not in the routing table 4 Ignores 19216820 from RouterB because RouterB has a hop count of 1 while the current routing table entry has a metric of 0 RouterB is saved for last, since it presents a more complicated situation: it is receiving routes from both RouterA and RouterB Here are the steps RouterB goes through: 1 Receives networks 10000 and 19216810 from RouterA and 19216820 and 19216830 from RouterC and increments the metric by one hop 2 Compares the advertised routes from RouterA and RouterC to what it has in its routing table 3 Adds 10000 and 19216830 because they are not currently in the routing table 4 Ignores 19216810 and 19216820 from RouterA and RouterC, respectively, because RouterA and RouterC have a metric of 1 for these routes, while the current routing table entries have a metric of 0 Looking at Figure 15-2, have the routers converged Remember the definition of convergence: the routers understand the complete topology of the network Given this definition, the routers have not yet converged RouterA s routing table
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