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| 6.4 |  | Troubleshooting Complex Network Systems |
| 6.4.2 |  | Disassembling the problem |

A good way to solve a complex problem is one piece at a time. If transport layer connectivity is failing, it may be because of a misconfiguration on more than one device or technology. Following the traffic flow and correcting each problem as it is encountered can be a valid troubleshooting approach. Examining the symptoms and generating a list of possible causes is less likely to be a successful troubleshooting methodology when dealing with more complex issues.

Even though it is a good practice to reverse changes that have no effect when troubleshooting simple problems, this is unlikely to help solve complex problems. With complex problems, the network engineer has to start relying on their own experience and judgment as to what is a probable cause of a problem and what is not.

Mechanisms for disassembling complex problems include gathering information from midpoints in the network communications chain and disabling parts of the system to exclude them as being the cause. This can involve recabling components of the network in order to insert monitoring hosts or other troubleshooting tools, or to bypass suspected equipment. Gathering detailed log information from key points in the communications chain can also help pinpoint specific problem areas.

When resolving complex network problems, the network engineer should always keep a record of changes being made. Keeping a log has the advantage of providing a record in case the configuration changes need to be reversed. A log of activities performed also removes any doubt as to whether a certain activity has been performed, helping the network engineer avoid repeating troubleshooting activities.