Troubleshooting Commands
207
debug Commands
The debug commands and options are very powerful tools. The messages
produced by the debugging process give detailed information and provide
insight into what is happening on a very low level.
This power does not come free of charge. Debugging requires every packet to
be process-switched, meaning that the route processor has to look at every packet
entering the router, if valid information is to be obtained. In addition, there
are many other processes the router must run and manage. Debugging can cause
a great deal of additional overhead on a router. Therefore, it is important to use
the tool with discretion. It should be used to provide additional information on an
existing problem, not used to monitor a router.
Because the majority of problems are reported while the network is in
production, the last thing you want to do is crash a router or cause unnec-
essary overhead by using the debug tool. By focusing the application of the
debug
command by using various options and access lists, you can effec-
tively troubleshoot problems without causing any additional ones. Always
remember to turn the debugging function off after you obtain the necessary
data. If left on, it can cause another network problem.
As with the show commands, there are global-, interface-, and protocol-
debugging options. Because these tools/commands are used in detail in
upcoming chapters, they are only summarized here according to usage.
Global Debugging
There are two "tricks" to successfully using the debug tool. First, make sure
that your router is configured to apply timestamps to all messages. This is
done with the following commands:
Router_A(config)#service timestamps debug datetime msec
localtime
Router_A(config)#service timestamps log datetime msec
localtime
Next, make sure that you see these messages. By default, error and debug
messages are sent only to the console. If you are telneted to the router, you do
not see the debug or log messages unless you issue the following command:
Router_A#terminal monitor
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