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Troubleshooting Methods for Cisco IP Networks


  1. Troubleshooting Principles
  2. Structured Troubleshooting Approaches
  3. Troubleshooting Example Using Six Different Approaches
  4. Summary
  5. Review Questions

Chapter Description

This chapter from Troubleshooting and Maintaining Cisco IP Networks (TSHOOT) Foundation Learning Guide: (CCNP TSHOOT 300-135) defines troubleshooting and troubleshooting principles. Next, six different troubleshooting approaches are described. The third section of this chapter presents a troubleshooting example based on each of the six troubleshooting approaches.

Troubleshooting Example Using Six Different Approaches

An external financial consultant has come in to help your company’s controller with an accounting problem. He needs access to the finance server. An account has been created for him on the server, and the client software has been installed on the consultant’s laptop. You happen to walk past the controller’s office and are called in and told that the consultant can’t connect to the finance server. You are a network support engineer and have access to all network devices, but not to the servers. Think about how you would handle this problem, what your troubleshooting plan would be, and which method or combination of methods you would use.

What possible approaches can you take for this troubleshooting task? This case lends itself to many different approaches, but some specific characteristics can help you decide an appropriate approach:

  • You have access to the network devices, but not to the server. This implies that you will likely be able to handle Layer 1–4 problems by yourself; however, for Layer 5–7, you will probably have to escalate to a different person.
  • You have access to the client device, so it is possible to start your troubleshooting from it.
  • The controller has the same software and access rights on his machine, so it is possible to compare between the two devices.

What are the benefits and drawbacks of each possible troubleshooting approach for this case?

  • Top-down: You have the opportunity to start testing at the application layer. It is good troubleshooting practice to confirm the reported problem, so starting from the application layer is an obvious choice. The only possible drawback is that you will not discover simple problems, such as the cable being plugged in to a wrong outlet, until later in the process.
  • Bottom-up: A full bottom-up check of the whole network is not a very useful approach because it will take too much time and at this point, there is no reason to assume that the network beyond the first access switch would be causing the issue. You could consider starting with a bottom-up approach for the first stretch of the network, from the consultant’s laptop to the access switch, to uncover potential cabling problems.
  • Divide-and-conquer: This is a viable approach. You can ping from the consultant’s laptop to the finance server. If that succeeds, the problem is most likely at upper layers. For example, a firewall or access control list could be the culprit. If the ping fails, assuming that ping is not blocked in the network, it is safe to assume that the problem is at network or lower layers and you are responsible for fixing it. The advantage of this method is that you can quickly decide on the scope of the problem and whether escalation is necessary.
  • Follow-the-path: Similar to the bottom-up approach, a full follow-the-path approach is not efficient under the circumstances, but tracing the cabling to the first switch can be a good start if it turns out that the link LED is off on the consultant’s PC. This method might come into play after other techniques have been used to narrow the scope of the problem.
  • Compare-configurations: You have access to both the controller’s PC and the consultant’s laptop; therefore, compare-configurations is a possible strategy. However, because these machines are not under the control of a single IT department, you might find many differences, and it might therefore be hard to spot the significant and relevant differences. The compare-configurations approach might prove useful later, after it has been determined that the problem is likely to be on the client.
  • Swap-components: Using this approach alone is not likely to be enough to solve the problem, but if following any of the other methods indicates a potential hardware issue between the consultant’s PC and the access switch, this method might come into play. However, merely as a first step, you could consider swapping the cable and the jack connected to the consultant’s laptop and the controller’s PC, in turn, to see whether the problem is cable, PC, or switch related.

Many combinations of these different methods could be considered here. The most promising methods are top-down or divide-and-conquer. You will possibly switch to follow-the-path or compare-configurations approach after the scope of the problem has been properly reduced. As an initial step in any approach, the swap-components method could be used to quickly separate client-related issues from network-related issues. The bottom-up approach could be used as the first step to verify the first stretch of cabling.