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9.3 | |
Troubleshooting Router Issues Overview | |
| 9.3.7 | |
Introduction to debug |
The
debug
commands assist in the isolation of protocol and configuration
problems. The debug
command is used to display dynamic data and events. Since the
show
commands only display static information, they provide a historical
picture of the router operation. Using the
debug
command output gives more insight to the current events of the router.
These events could be traffic on an interface, error messages
generated by nodes on the network, protocol-specific diagnostic
packets, and other useful troubleshooting data.
 The
dynamic output of the
debug command comes at a
performance cost, producing high processor overhead that may disrupt
normal router operation. For this reason,
debug
should be used conservatively. Use
debug
commands to examine specific types of traffic or problems after likely
problems have been narrowed a few causes.
debug
should be used isolate problems and not to monitor normal network
operation.
By default, the router sends the debug output and system messages to the console. If a telnet session is being used to examine the router, then the debug output and system messages can be redirected to the remote terminal. This is done through the telnet session by issuing the terminal monitor command. Extra caution should be used when selecting the debug commands from a telnet session. No command should be selected that will cause the debug output to create additional traffic that creates debug output. If this occurs, the telnet session will rapidly saturate the link with traffic or the router will exhaust one or more resources. A good rule to follow to prevent this recursion of traffic is “Never debug any activity on the port where the session is established”. The output of the different
debug
commands varies. Some may frequently generate many lines while others
produce a line or two of output every few minutes.
Another IOS software service that will enhance the usefulness of the debug output is the timestamps command. This command will put a timestamp on a debug message. This information provides the time when the debug event occurred and the duration of time between events. The following command configures a timestamp that will show the hour:minute:second of the output, the amount of time since the router was last powered up, and when a reload command was executed:
The no debug all command or the undebug all command turn off all diagnostic output. To disable a particular debug command, use the “no” form of the command. For example, if debug for monitoring RIP was enabled with the command debug ip rip, it can be disabled with no debug ip rip. To view what is currently being examined by a debug command, use show debugging.
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