qr code c# sample Problem: Routing Loops in Objective-C

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Problem: Routing Loops
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The other main problem of distance vector protocols is that they are prone to routing loops A routing loop is a layer 3 loop in the network Basically, it is a disagreement about how a destination network should be reached
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To verify whether a routing loop exists, use the ping tool to test connectivity to the destination network:
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if you receive a reply of TTL expired in transit for the destination, you know a routing loop exists (TTL is time-to-live)
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Routing Loop Example
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Let s take a look at a simple example of the kind of problems routing loops can create Use the network shown in Figure 15-7 In this example, assume that RouterX was
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Distance Vector Protocol Problems and Solutions
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originally advertising 19216840 to RouterA, which passed this on to RouterB RouterX, though, has failed and is no longer advertising 19216840 RouterA will eventually learn this by missing routing updates from RouterX RouterA then incorporates the change into its routing table RouterA must then wait for its periodic timer to expire before forwarding this update to RouterB Before this happens, however, RouterB advertises its routing table to RouterA, which includes the 19216840 route, making it appear to RouterA that this network is reachable via RouterB Since both RouterA and RouterB advertise 19216840 to each other, this creates confusion about how to reach 19216840, if it can even be reached (and in this case, it can t) In this example, RouterA thinks that to reach 19216840, it should send these packets to RouterB RouterB, on the other hand, thinks that to reach 19216840, it should use RouterA This is a very simple example of a routing loop Typically, routing loops are created because of confusion in the network related to the deficiencies of using periodic timers Distance vector protocols use several mechanisms to deal with routing loop problems However, these solutions slow down convergence Link state and some hybrid protocols deal with routing loops better by using more intelligent methods that don t slow down convergence The following sections cover the methods that a distance vector protocol might implement to solve routing loop problems
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FIGURE 15-7
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Simple routing loop example
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RouterX 19216810 E0
RouterA
19216820 E1 E0
RouterB E1
RouterA s Routing Table Network Interface Metric 19216840 19216810 19216820 19216830 19216840 E0 E1 E1 E1 0 0 1 2
RouterB s Routing Table Network Interface Metric 19216820 19216830 19216810 19216840 E0 E1 E0 E0 0 0 1 2
15: Routers and Routing
Counting to Infinity Solution: Maximum Hop Count
One problem with a routing loop is called the counting to infinity symptom When a routing loop occurs and a packet or packets are caught in the loop, they continuously circle around the loop, wasting bandwidth on the segments and CPU cycles on the routers that are processing these packets To prevent packets from circling around the loop forever, distance vector protocols typically place a hop count limit as to how far a packet is legally allowed to travel As a packet travels from router to router, a router keeps track of the hops in the TTL field in the IP datagram header: for each hop a packet goes through, the packet s TTL field is decremented by 1 If this value reaches 0, the packet is dropped by the router that decremented the value from 1 to 0 (The function of the TTL field was covered in 6) Placing a maximum hop count limitation on packets, however, doesn t solve routing loop problems the loop still exists This solution only prevents packets from getting stuck in the loop Another issue with placing a hop count limit on packets is that, in some instances, the destination that the packet is trying to reach exceeds the maximum hop count allowed A router doesn t distinguish between valid destinations and routing loop destinations when examining the TTL field; if the maximum is reached, then the packet is dropped RIP sets a hop count limit of 15 When a packet comes into an interface of a router, it decrements the TTL field, and if the hop count falls to 0, the router immediately drops the packet If you have a destination that is beyond these limits, you can change the maximum hop count for your routing protocol; however, you should do this on every router in your network
Routing loops are a misunderstanding regarding the reachability of a destination and are a common problem with distance vector protocols Different solutions are used to
deal with routing loops and their problems Counting to in nity is dealt with by assigning a hop count limit to restrict how far a packet can travel
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