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.
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.