Imagine trying to solve a network failure by using a different approach every time. With today’s
complex networks, the possible scenarios would be innumerable. Because so many different
things can go wrong within a network, it’s possible to start from many different points. Not
only is this an ineffective method of troubleshooting, but it’s also time-consuming, and time is
very valuable in a “network down” situation.
Cisco has designed an effective
troubleshooting model
that contains three steps. A troubleshooting
model is a list of troubleshooting steps or processes that can be followed to provide an
efficient manner of resolving network problems. The headings in this section contain information
specific to each step of the troubleshooting model. After the three steps are completed and the
problem is resolved, a few more actions follow, such as documenting the problem-solving events.
To be effective when troubleshooting and to achieve faster resolution times, follow the model
outlined in Figure 33.2. This flow chart shows the three steps.
The troubleshooting process begins when a network failure is reported to you. The following
are brief descriptions of the steps to take:
1.
Gather symptoms.
At this point in the process, it is important gather and document the
symptoms of the problem that is being experienced.
2.
Isolate the problem.
After identifying the symptoms, the administrator looks for commonalities
in the symptoms and tries to determine at what layer of the OSI model the problem
is occurring. During this phase, it may be necessary to go back and gather more symptoms.
3.
Correct the problem.
Based on the information that was gathered and the determinations that
were made in the previous two steps, the network administrator now makes the changes necessary
to correct the problem. Once the corrective steps have been taken, the administrator
observes the results of the changes to ensure that the problem was corrected. If the problem
was not corrected, then the changes made should be backed out and the administrator should
start the troubleshooting process over with the gather symptoms stage. If the changes do correct
the problem, then the administrator should update the necessary documentation. The
final item of importance when correcting the problem is to make sure that you make only one
change at a time. This will ensure that you do not make unnecessary changes, which could
introduce new problems.
The best way to understand how Cisco’s model works and how you should use it is by looking
at an example. For this example, assume you are in charge of operational support of the network
pictured in Figure 33.3. There are two campus networks, connected via a Frame Relay
cloud. Within each network, VLANs are connected to a Catalyst 6500 switch and then to a core
router that has a connection to the Frame Relay cloud in one way or another.
The fun begins when you get a call from a user who “can’t get to Host Z.” Based on this
information, let’s apply Cisco’s troubleshooting model to solve the user’s difficulty and fix the
problem in the network.