This step within the troubleshooting model is used to contemplate the possible causes of the failure.
Obviously, it is quite easy to create a very long list of possible causes. That’s why it’s so
important to gather as much relevant information as you can in the gathering symptoms phase.
By defining the problem and assigning the corresponding boundaries, the resulting 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’t ping to anywhere outside its own network.
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.
Based on what you know, you now need to list possible causes. These possible causes are
as follows:
No default gateway is configured on Host A.
The wrong subnet mask is configured.
There is a misconfigured access list on the router connected to the switch on Campus A.
If you had not gathered such specific information in step 1, this list could have included all
possible problems with any piece of equipment between Host A and Host Z. That would have
been a long list, and it would take a lot of time to eliminate all of the possible causes.
Remember that because these are only
possible
causes, you still have to choose the most
likely option, implement it, and observe to see whether the changes made were effective. When
the list of possible problems is long, it may require more iterations of the problem-solving steps
to actually solve the problem. In this example, you have only three possible causes, so this is a
much more manageable list. Although there may be other possible causes that you can think of
(and it’s great that you can do that), for this example and in the interest of simplicity, only these
three are listed.
Here’s where it gets interesting. You now have to check each of these possibilities and fix
them if they are the cause of the problem.