Constructing CCNA Config Drills for Troubleshooting Practice

By certskills December 11, 2013 09:05

To answer a Sim question on the #CCNA exam, you need skills. Obviously, you need troubleshooting skills, because Sim questions generally start with a non-working network, and your job is to configure something to fix the problem. But you need a not-so-obvious skill as well: solid configuration skills. Today’s post continues a new troubleshooting series, showing an example exercise that helps you prepare your configuration skills.

Earlier posts in this series:

This is the first one!

Sim Question on Exam Day Vs. Prep

A Sim question on the exam typically doesn’t give you a lot of info. It’s often just a network diagram, some symptom (A can’t ping B, for instance), and a goal: fix the config. You get the points if you change/add to the config to overcome the problem.

As discussed in the previous post in this series, it’s hard to give yourself a
troubleshooting exercise when doing self-study. If you create the problem for
yourself, you can’t help but remember what problem you created. Today’s post gives an example of an exercise that:

  • You can make up these exercises for yourself without spoiling the answer
  • The exercises help you master how to mentally configure the devices correctly, which allows you to find differences between your mental correct configuration and the actual (incorrect) config in a Sim question.

The rest of the post details one such exercise as an example.

Info You Probably Get in the Sim Question

Sim questions will typically give you a topology diagram, because the user interface gives you access to the CLI (through the console) by clicking on devices in the diagram. So, Sim questions at least supply the topology. In some cases, you might also see the interface type/number listed. Figure 1 shows an example with both types of information.

Figure 1: Sample Topology


Sim questions also typically give you some idea what is wrong. For this exercise, assume that host A cannot ping host B.

Info to Choose for Your Own Configuration Exercise

Could you create a configuration for the three routers and switches in Figure 1? And assign IPv4 addressing details to the three hosts? Sure, if you made a lot of assumptions. However, if I stopped this blog post now, and asked you to configure the network in figure 1, and submit your answers, many of your answers would be different.

To create this kind of config exercise at home (with a goal of helping you in troubleshooting), start with a network topology, like in Figure 1. Then start making a few key choices about the design, as discussed in the rest of this post. Then you’ll have a new exercise.


First, you need some IPv4 addressing and subnets. Generically, you need to choose:

  1. An IPv4 network to use
  2. Whether to use one mask or many (VLSM)
  3. What mask(s) to use
  4. Some rules about how to assign subnets to the design

As an example, I’ll choose for this blog post:

  1. Use Class B network
  2. Use one mask
  3. Use mask
  4. Use the numerically lowest subnets possible where “subnet 1” is listed in Figure 2, next highest subnet ID value for subnet 2, and so on.

Figure 2

IPv4 Addresses

You need to pick some specific IPv4 addresses for each router, host, and switch as well. If you use your exercise only for yourself, just pick some numbers. However, if you plan to share with others that you study with, or post on the Cisco Learning Network, then make some rules so people know your intent for the individual addresses as well.

For example, the following convention is easy to follow in words:

  1. For routers, add the router number to the subnet ID. EG, if the subnet ID were, connected to R2, R2’s IP address on that interface would be
  2. For hosts, give them an IP address +1 compared to the router on the same LAN. EG, if R2’s address was on a LAN interface, and a host was on that same LAN, give the host
  3. For a switch’s IP address, use the numerically highest IP address in the subnet.

Routes and Routing Protocols

Continuing down this path of your making your own exercises, I think each exercise should use only one of the following: static routes, OSPF, or EIGRP. I’m choosing OSPF for this example, mainly to show how to deal with the wildcard mask choice. Here are the design choices I’m suggesting for today’s exercise:

  1. Use OSPF on all three routers
  2. Use a single area (area 0) for the entire design.
  3. Configure a recognizable router ID.
  4. On your network commands, use a wildcard mask that causes a match of all IP addresses in the same subnet. EG, an interface uses address, mask, use a network command that matches all IP addresses in subnet, mask

Other Settings

Some other settings may influence the correct answer to a Sim question. Here’s a brief list to consider while you build your own exercises and work on my samples:

Serial links: The interface to which the DCE end of the cable connect needs a clock rate command.

Passwords and other administrative settings: Ignore them, unless the Sim question you’re imagining implies that they matter. For example, if the question states that host A cannot Telnet to router R2, then imagine the correct configuration on R2 that would allow Telnet to work.

ACLs: Any Sim question that begins with “host A can’t communicate with host B” may have a root cause related to an ACL that filters the traffic.

Do this Exercise!

In the mean time, go ahead and practice! Take the criteria spelled out in this blog post, and create what you think are the correct configurations for routers R1, R2, and R3 in the two figures. I’ll post some answers, and then move on to how to use your answers to do some additional troubleshooting practice for Simlet questions.


#CCNA Tshoot Config Exercise - Answers
Answer to the #CCNA EIGRP Question
By certskills December 11, 2013 09:05
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