#CCNA Troubleshooting Self-study Exercises

The #CCNA exam, especially the #ICND2 half of the content, focuses on troubleshooting.  However, for self study candidates, practicing troubleshooting can be a challenge. This post discusses the challenge, one solution, and begins a series that gives examples of how to practice troubleshooting skills for CCENT and CCNA.

Earlier posts in this series:

This is the first one!

Troubleshooting Perspectives for CCENT and CCNA

Troubleshooting of anything – networks or otherwise – can often be reduced to the following kind of logic:

1. What should be happening?
2. What is happening, and in particular, what’s different compared to what should be happening?
3. What change can make the “thing” start working like it should?

For instance, say your car won’t start. It of course should start (what should happen), but when you turn the key, nothing happens. So, you have it towed to the auto repair shop, and they replace the starter, so the car starts again.

For CCENT and CCNA troubleshooting, you need to be ready to describe what should be happening, what is happening, and compare the two. Specifically, you should be ready to think about:

1. What should the configuration of each device be?
2. What is the current configuration of each device, and how is that configuration different from what I expect the configuration to be?
3. How will the devices work with the current configuration, that is, what problems does the current configuration cause?
4. What show commands reveal the current operation of the network, revealing the problem, particularly if I cannot see the current configuration?
5. What configuration changes would solve the problem?

You and Your CCENT and CCNA Troubleshooting Skills

If you personally look at the above list, and already feel confident about those skills for all the topics in the scope of CCNA, great! That’s where you need to be to be 100% prepared for the exam. For the rest of you, the next question becomes: How do I develop those skills?

Building troubleshooting skills when studying alone poses a difficulty. If you personally create a problem in a network, you already know the problem, and it spoils the answer, so to speak. One way to solve this problem is to find someone to study with. However, a few exercises can build these skills, even when working alone.

• Build the expected configuration for a working network
• Break the config and predict the show command output that results

To see why these help, first think back to the two types of CCNA questions that use a Simulator: Sim questions and Simlet questions.

Sim Questions Mentally Begin with the Expected Configuration

On Cisco exams, a Sim question gives you a network diagram, some existing configuration, with the expectation that you change/add to the configuration of some device to fix a problem. Mentally, you basically run through this process:

1. What is the current configuration?
2. What would I expect the configuration to be to make the network work?
3. What’s different – and therefore, what should a change/add to the config?

You can practice for #2 on your own. Just take any small network diagram, make up some requirements, and create the correct configuration on paper. Feel a little unsure? Post your scenario on the Cisco Learning Network, and ask others to make comments about whether they think it’s correct.

Simlet Questions Require Puzzling w/ Show Commands

Simlet questions also give you a network diagram, some existing configuration, and access to the CLI from which to issue commands. However, you often do not even have access to the device configurations, that is, the show running-config command doesn’t work. Plus, you get several multiple-choice questions to answer about the current operation of the network. Your job: analyze the current operational status.

To prepare for Simlets, break your own configurations. That is, after creating a configuration for some network as just described, either delete one line of the configuration, or change one parameter. Then ask yourself: how will this network behave with the changed configuration? Which show commands will have different output than it had before? What will that output look like?

Of course, if you have a place to try this kind of experiment, it helps – a simulator, some real gear, and so on. But you can still do this kind of exercise on paper, particularly if you just delete change one command or parameter.

For example, if your configuration of router R1 formerly had:

Router ospf 1
  Network 10.1.1.0 0.0.0.255 area 0
  Network 10.1.2.0 0.0.0.255 area 1

And then you decide to delete the second network command, what is the result? What show commands differ, and how?

Examples on the Way!

To help you see what I mean, I’ll spend some time here in the blog creating a few examples. Next post, we’ll start with a scenario for creating the configuration you would expect to see for a small network.