other routers in the network One downside to hold-down timers is that they slow down convergence in Objective-C

Paint QR Code ISO/IEC18004 in Objective-C other routers in the network One downside to hold-down timers is that they slow down convergence

other routers in the network One downside to hold-down timers is that they slow down convergence
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Example of Route Poisoning and Hold-Down Timers
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Understanding how poisoned routes and hold-down timers work can become complex Let s take a look at an example to see how these two mechanisms work hand-in-hand to solve large routing loop problems Use the network shown in Figure 15-9 In this example, assume the routers are running RIPv1 In this example, RouterA s E0 interface fails, causing it to lose its connection to 19216810 Since RIPv1 doesn t use triggered updates, the routing protocol must wait for its periodic timer to expire before broadcasting its routing information to
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Distance Vector Protocol Problems and Solutions
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FIGURE 15-9
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Route poisoning and hold-down timer example
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RouterB and RouterC In RIPv1, the periodic update timer is set to 30 seconds RouterA will poison the route (assign an infinite metric of 16 to 19216810) and send this to the other two routers when the periodic update timer expires When RouterB and RouterC receive the routing update with the poisoned route from RouterA, they will send back a poison reverse to RouterA All routers will freeze the poisoned route in their routing tables for the period of the hold-down timer In RIPv1, this defaults to 180 seconds RouterB and RouterC also advertise the poisoned route in their routing updates out any other active interfaces (once their periodic timers expire) As the propagation of the poisoned route is occurring, the routers that have already received it are counting down from their hold-down timer value If another router in the network advertises a worse path to 19216810 (this has to be a worse hop count than the route originally advertised from RouterA), the three routers shown in the network diagram won t use it, since they have frozen the poisoned route in their routing tables The reason for this hold-down period is that someone else might be advertising 19216810, but it might not be a valid path In other words, another router might be advertising reachability to 19216810, but it is assuming that this network is reachable via RouterA In this situation, this rogue router hasn t received the poisoned route the hold-down timer for the other routers, however, ensures that these rogue routers don t corrupt the routing tables by introducing incorrect or bad routing information, causing a routing loop
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15: Routers and Routing
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During this process, if RouterA is able to fix its connection to 19216810, it will start advertising the reachability of the network to RouterB and RouterC Since the metric RouterA is advertising is the same as the metric it had previously announced for this route, RouterB and RouterC will cancel their hold-down times and replace the poisoned route with the new information
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INSIDE THE EXAM
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Routing Introduction
Understand the basic functions of routers and routed and routing protocols Knowing the terms in this chapter is very important: know what an autonomous system is, what administrative distance is, and what a metric is Remember the administrative distances of the different IP routing protocols be able to pick out what route will show up in the routing protocol based on multiple routing protocols learning the same route compare distance vector and link state protocol characteristics Know how link state protocols operate through the use of LSAs and how they build a routing table
Distance Vector Protocol Problems
Understand the problems with distance vector protocols and the solutions employed to solve them Understand how to find a routing loop by examining a routing table and the solutions used to solve routing loop problems: counting to infinity, split horizon, hold-down timers, poisoned routes, and poisoned reverse Be familiar with these terms and know what they mean
Dynamic Routing Protocols
Remember the metric components that routing protocols use Be familiar with how distance vector protocols operate using the Bellman-Ford algorithm, and be able to
CERTIFICATION SUMMARY
Routers find layer 3 paths to destination networks and switch packets from one interface to another to get the packets to their respective destinations Routers learn about neighboring routers, find locations to destination locations, choose the best paths, and maintain up-to-date routing information A routed protocol is a layer 3 protocol, such as IP or IPX A routing protocol defines how to find destinations for a routed protocol, such
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