IOS processes, protocols, mechanisms and events generate messages to communicate their status. These messages can provide valuable information when troubleshooting or verifying system operations. The IOS debug command allows the administrator to display these messages in real-time for analysis. It is a very important tool for monitoring events on a Cisco IOS device.
All debug commands are entered in privileged EXEC mode. The Cisco IOS allows for narrowing the output of debug to include only the relevant feature or sub-feature. This is important because debugging output is assigned high priority in the CPU process and it can render the system unusable. For this reason, use debug commands only to troubleshoot specific problems. To monitor the status of ICMP messages in a Cisco router, use debug ip icmp.
To list a brief description of all the debugging command options, use the debug ? command in privileged EXEC mode at the command line.
To turn off a specific debugging feature, add the no keyword in front of the debug command:
Router# no debug ip icmp
Alternatively, you can enter the undebug form of the command in privileged EXEC mode:
Router# undebug ip icmp
To turn off all active debug commands at once, use the undebug all command:
Router# undebug all
Some debug commands such as debug all and debug ip packet generate a substantial amount of output and use a large portion of system resources. The router would get so busy displaying debug messages that it would not have enough processing power to perform its network functions, or even listen to commands to turn off debugging. For this reason, using these command options is not recommended and should be avoided.
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