Networking, Routers and Routing


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     Troubleshooting Networks


    Troubleshooting And Solving Network Problems

    These notes explain some of the commands that can be used for troubleshooting and solving network connectivity problems.

    Introduction

    The Internetwork

    Troubleshooting Example 1
            The First Problem

    Troubleshooting Example 2
            The Second Problem

    Troubleshooting Example 3
            The Third Problem

    Summary

     
     

    Introduction

    Suppose you are a network technician and your network starts experiencing problems, with some networks unable to connect to other networks. What would be your approach to diagnosing and solving the problems? Well, to successfully hone in on a problem, then diagnose and solve it requires a systematic approach. This will come through experience and usually boils down to a process of elimination. One of the best starting tools in the elimination process are the ping and tracereoute commands. They will help you figure out which particular network device is experiencing problems. If the problem device happens to be a router, then there are a variety of diagnostic commands you can run that will help you identify the particular problem and allow you to correct it.

    On completion of these notes you should be able to...

    • Test connectivity of a network using ping a traceroute.
    • Diagnose problems using various router commands
    • Apply a systematic approach to troubleshooting networks

    The Internetwork

    The diagram bellows shows the Internetwork we are going to troubleshoot.

    There are 5 separate LANs connected together through 6 routers. From the bottom, we have the Southwark LAN, the London LAN, the Paris LAN, the Sydney LAN and the New York LAN.

    Zoom Top Left        Zoom Top Right

    Zoom Original

    Zoom Bottom Left        Zoom Bottom Right

     

    Troubleshooting Example 1

    There are a number of problems on this particular internetwork. At the moment it is a mystery, but some networks are unable to connect to other networks. Why? Well that's what we have to find out. Now, troubleshooting network problems usually starts with a lot of pinging and tracing routes. This helps identify which routes are broken and starts the process of elimination. There are also a number of router commands that can be used to narrow down the problem even further and help diagnose the problem. However, in the end, just like anything, being able to solve problems on a network takes practice and experience, just like anything else.

    The First Problem

    On this example internetwork, I am going to start the troubleshooting process by trying to connect PC2 on the Southwark LAN to PC1 on the Paris LAN. Activity A below shows the connectivity testing and troubleshooting process step-by-step.

    ~~Activity~~

     Activity A

    1. Test a Route       2. Tracert PC2       3. Ping Gateway       4. Hypothesis

    5. sh ip int brief        6. Check Documentation        7. Solution        8. Verification

     

    Troubleshooting Example 2

    Now that the first problem has been corrected, it is time to move on and test other routes on the internetwork.

    The Second Problem

    Although the problem with the Southwark gateway has now been solved and PC2 can ping it's own gateway, the question now is, can PC2 ping PC1 on the Paris LAN? That was the original intention after all. Activity B answers that question.

    ~~Activity~~

     Activity B


    1. Test a Route       2. Ping London        3. Ping Paris        4. Hypothesis

    5. sh ip int brief        6. sh controllers serial        7. Solution        8. Verification

     

    Troubleshooting Example 3

    Well, that's two problems sorted out. Now for the final problem on this internetwork.

    The Third Problem

    So now for the third problem, which is a trickier problem compared to the other two. As you can see from the initial diagram in Activity C below, the problem lies somewhere between PC2 on Southwark and PC6 on New York. You might think that's a lot of routers to check in between, but the problem can be narrowed down to a particular router fairly quickly using ping or traceroute.

    ~~Activity~~

     Activity C

    1. Test a Route       2. Tracert PC2       3. Ping New York        4. Hypothesis

    5. sh ip route        6. sh ip protocols        7. Solution        8. Verification

      

    Summary

    On completing these notes you should have learned the following key points:-

    • The ping and traceroute commands can be used to test connectivity on a network
    • A router has a number of commands that can help you with diagnosing problems
    • The sh ip int brief command is useful for checking the status of router interfaces
    • The sh controllers serial command is useful for determining if a serial cable is attached and which end is the clocking DCE end of the cable and which is the DTE end.
    • The sh ip route command is useful for checking that networks that should be in the routing table are actually in the table; e.g. directly connected networks and also remote networks discovered through a routing protocol or static route.
    • The sh ip protocols command is useful for checking that the routing protocol and network statements are correct.
     
     

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