1.3.b Apply troubleshooting methodologies

1.3.b Apply troubleshooting methodologies
     1.3.b (i) Diagnose the root cause of networking issue (analyze symptoms, identify and describe                          root cause)
     1.3.b (ii) Design and implement valid solutions according to constraints
     1.3.b (iii) Verify and monitor resolution

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• Gathering information: Gathering information happens after the problem has been reported by the user (or anyone). This might include interviewing all parties (user) involved, plus any other means to gather relevant information. Usually, the problem report does not contain enough information to formulate a good hypothesis without first gathering more information. Information and symptoms can be gathered directly, by observing processes, or indirectly, by executing tests.
• Analyzing information: After the gathered information has been analyzed, the troubleshooter compares the symptoms against his knowledge of the system, processes, and baselines to separate normal behavior from abnormal behavior.
• Eliminating possible causes: By comparing the observed behavior against expected behavior, some of the possible problems causes are eliminated.
• Formulating a hypothesis: After gathering and analyzing information and eliminating the possible causes, one or more potential problem causes remain. The probability of each of these causes will have to be assessed and the most likely cause proposed as the hypothetical cause of the problem.
• Testing the hypothesis: The hypothesis must be tested to confirm or deny that it is the actual cause of the problem. The simplest way to do this is by proposing a solution based on this hypothesis, implementing that solution, and verifying whether this solved the problem. If this method is impossible or disruptive, the hypothesis can be strengthened or invalidated by gathering and analyzing more information.

A good problem description consists of accurate descriptions of symptoms and not of interpretations or conclusions. Consequences for the user are strictly not part of the problem description itself, but can be helpful to assess the urgency of the issue. When a problem is reported as "The mail server isn't working," you must perhaps contact the user and find out exactly what he has experienced. You will probably define the problem as "When user X starts his e-mail client, he gets an error message saying that the client can not connect to the server. The user can still access his network drives and browse the Internet."

• Top down: Using this approach, you work from the Open Systems Interconnection (OSI) model's application layer down to the physical layer.
• Bottom up: The bottom-up approach starts from the OSI model's physical layer and moves up to the application layer.
• Divide and conquer: Using this approach, you start in the middle of the OSI model's stack (usually the network layer) and then, based on your findings, you move up or down the OSI stack.
• Follow the path: This approach is based on the path that packets take through the network from source to destination.
• Spot the differences: As the name implies, this approach compares network devices or processes that are operating correctly to devices or processes that are not operating as expected and gathers clues by spotting significant differences. In case the problem occurred after a change on a single device was implemented, the spot-the-differences approach can pinpoint the problem cause by focusing on the difference between the device configurations, before and after the problem was reported.
• Move the problem: The strategy of this troubleshooting approach is to physically move components and observe whether the problem moves with the components.