Troubleshooting guide

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Cisco Broadband Local Integrated Services Solution Troubleshooting Guide
OL-5169-01
Chapter 2 Troubleshooting Overview
Troubleshooting Strategy
One of the most important outcomes of a systematic troubleshooting approach is to narrow possibilities
—remove irrelevant details from the set of items that you need to check. You can eliminate entire classes
of problems associated with system software and hardware. You can eliminate several possible causes
based on the facts gathered for the problem. Consider the possible causes identified earlier and eliminate
those that are not relevant to narrow the scope of the problem as much as possible.
Step 4: Create an Action Plan
You can now devise an action plan based on the set of possibilities that were just created. From these
possibilities, you can implement a "divide and conquer" policy. Consider the most likely possibility, and
determine a plan in which only one variable is manipulated. This approach allows you to reproduce a
given solution to a specific problem. If more than one variable is altered simultaneously and the problem
is solved, you cannot identify which variable caused the problem.
• Use a partitioning effect. Split your troubleshooting domain into discrete areas that are logically
isolated from each other. This approach allows you to determine which side of the partition (if not
both sides) keeps the problem after the partitioning.
• Determine where in the network the problem exists. Use a series of tests to pinpoint where network
failure occurs. Begin from a source device and try a sequence of tests to determine whether proper
functioning occurs from the source to successively more distant, intermediate network devices. This
approach allows you to gradually trace a path from a source along the way to the ultimate destination
and possibly isolate the part of the path that contains the problem.
• Collaborate with others and share rules-of-thumb action plan approaches. The more of these logical
problem-solving approaches you learn, the more tools you have. Tools help you test a given problem
situation. As you gain experience, you improve your ideas on how to relate the given possibilities
and your troubleshooting tools to a specific and systematic action plan.
An action plan for a typical cause, for example, is to look at each device's current configuration and
determine whether any recent changes were accomplished correctly.
Step 5: Implement the Action Plan
It is important to be very specific in creating and executing an action plan; the plan must identify a set
of steps to be executed, and each step must be carefully implemented. Keep track of exactly what you
are testing. Try not to change too many variables at the same time.
As you implement your action plan, also try to
• Make sure what you implement does not make the problems worse or add new problems.
• Limit as much as possible the invasive impact of your action plan on other network users.
• Minimize the extent or duration of potential security lapses during your action plan implementation.
It is important to have a backout plan (for example, a saved configuration file) to return the network to
a known previous state. For example, connect to a device's command console to view its configuration.
If the configuration is deemed to be incorrect, reconfigure the device or temporarily disable it.
To ensure that not more than one variable is manipulated at a time, the results of the changes made must
be observed before any changes are made to the configuration of this device or any other device.