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Appendix C. Scenario Questions

The Cisco Support exam scenario questions aren't easy. You won't be shown a picture of three routers ”two of them shiny and new and the third smoking ”and asked to choose which router has a problem. You may, instead, get a single description of a series of problems experienced by a very large network that connects routers and switches located across many cities. After reading the given scenario, you're asked to determine the source of the problems.

The following are three common scenarios similar to those you may encounter on the exam.

Scenario 1”Bad Subnetting

In Figure C.1, you will see a hub and spoke network. There are two subnetted IP addresses. One address is there to throw you off and is invalid. Do you know subnetting well enough to tell which link has an invalid subnet?

Figure C.1. A diagram showing invalid subnetting.

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The link between Atlanta and Denver would be the problem link. The reason is that a /27 network is the same as stating a subnet of 255.255.255.224 . If you take a magic number of 256 and subtract 224 , you get 32 . This means that the first network would be the .32 network, then second the .64 network. The broadcast addresses in the .32 network would be .63 and all the addresses in between would be valid. This means that if the interface in Denver was 172.16.1.33 , the only valid addresses in that network would be from .34 to .62 . Only those addresses could be used on the Atlanta side of the link.

Scenario 2” Utilizations

In Figure C.2, you are looking at routers in another hub and spoke network. The administrators of the routers in these four cities have recently used the show process cpu command on the routers and determined their average utilization. Looking at the utilization percentages of these four routers, which of them needs to be upgraded?

Figure C.2. The Average router utilizations from routers in four cities.

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A router that is slow on its processing times is basically a better doorstop than a router in your network. Depending on the type of router, Cisco recommends that a router operating above 65 to 70 percent average utilization needs to be upgraded. If you go by the Cisco recommendations, this means that both the routers in Atlanta and Boise need to be upgraded or replaced .

Scenario 3”Determining the Probable Source

This is the type of scenario you can expect to see reflected in several questions on the exam. You should understand how ICMP works in the network as a troubleshooting tool. In Figure C.3, you see a network with five PCs on different floors. There is a connectivity issue and PCs on the first floor cannot communicate with PCs on the second floor.

Figure C.3. A network where the first floor PCs have problems communicating with the PCs on the second floor.

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The two PCs on the first floor can ping each other, but they cannot ping their own configured default gateway. The PCs on the second floor can successfully ping each other and their default gateway. The PCs on the second floor cannot ping the IP address of the first floor PCs' default gateway. What is the most likely problem?

In this case, the most likely problem could be narrowed down to either a misconfigured routing table on the router, an interface configuration problem, or a hardware problem with the Fast Ethernet 0/0 interface. One or more of these items would be listed as choices for correct answers on the exam.

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Appendix C. Scenario Questions