General Troubleshooting Model
When you’re troubleshooting a network environment, a systematic approach works best. Define
the specific symptoms, identify all potential problems that could be causing the symptoms, and
then systematically eliminate each potential problem (from most likely to least likely) until the
symptoms disappear.
Figure 13-7 illustrates the process flow for the general problem-solving model. This process
flow is not a rigid outline for troubleshooting an internetwork; it is a foundation from which you
can build a problem solving process to suit your particular environment.
Figure 13-7 General Problem Solving Model
The following steps detail the problem solving process outlined in Figure 13-7:
Step 1 When analyzing a network problem, make a clear problem
statement. You should define the problem in terms of a set of
symptoms and potential causes. To do this, identify the general
Define the problem
Gather the facts
Create an action plan
Implement the action plan
Observe the results
Repeat the process
Problem resolved; terminate the process
(If symptoms stop...)
(If symptoms persist...)
Consider possibilities based on the facts
General Troubleshooting Model 445
symptoms and then ascertain what kinds of problems (causes)
could result in these symptoms. For example, hosts might not be
responding to service requests from clients (a symptom). Possible
causes might be a misconfigured host, bad interface cards, or
missing router configuration commands.
Step 2 Gather the facts you need to help isolate possible causes. Ask
questions of affected users, network administrators, managers,
and other key people. Collect information from sources such as
network management systems, protocol analyzer traces, output
from router diagnostic commands, or software release notes.
Step 3 Consider possible problems based on the facts you gathered.
Using the facts you gathered, you could eliminate potential
problems from your list. For example, depending on the data, you
might be able to eliminate hardware as a problem allowing you to
focus on software problems. At every opportunity, try to narrow
the number of potential problems so you can create an efficient
plan of action.
Step 4 Create an action plan based on the remaining potential problems.
Begin with the most likely problem and devise a plan in which
only one variable is manipulated. This approach allows you to
reproduce a given solution to a specific problem. If you alter more
than one variable simultaneously, you might solve the problem,
but identifying the specific change that eliminated the symptom
becomes more difficult.
Step 5 Implement the action plan, performing each step carefully while
testing to see if the symptom disappears.
Step 6 Whenever you change a variable, be sure to gather results.
Generally, you should use the same method of gathering facts that
you used in Step 2. Analyze the results to determine whether the
problem has been resolved. If it has been resolved, then the
process is complete.
Step 7 If the problem has not been resolved, you must create an action
plan based on the next most likely problem in your list. Return to
Step 4 and reiterate the process until the problem is solved. Make
sure to undo any “fixes” you made in implementing your action
plan. Remember that you want to change only one variable at a
time.
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