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Chapter 1

Troubleshooting Methodology

You now have isolated the problem to anything outside the immediate

network. Upon further inspection and fact gathering, you find that the user
can't ping external hosts, either. With all of this information, you can now
start to contemplate possible causes of this failure and move on to the
following Consider Possibilities section.

Using Baseline Information

Baseline information can be a great asset to troubleshooting network prob-
lems. Baseline information includes historical data about the network and
routine utilization information. This information can be used to determine
whether there were recent changes made to the network that may contribute
to the problem at hand.

Step 3: Consider Possibilities

This step within the troubleshooting model is used to contemplate the pos-
sible causes of the failure. Obviously, it is quite easy to create a very long list
of possible causes. That is why it is so important to gather as much relevant
information as you can and to create an accurate problem statement. By
defining the problem and assigning the corresponding boundaries, the result-
ing list of possible causes diminishes because the entries in the list will
be focused on the actual problem and not on "possible" problems.

First, review what you know about your sample problem:

Host A can't FTP to Host Z.

Host A can't FTP to any host on Campus B.

Host A can FTP to any host on its own network.

All other hosts on Host A's network can FTP to Host Z, as well as to
other hosts.

Host A can't ping to anywhere outside its own network.

Based on what you know, you now need to list possible causes. These

possible causes are:

No default gateway is configured on Host A.

There is a duplicate IP address.

The wrong subnet mask is configured.

There is an inbound access list on the router connected to the switch
on Campus A.

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