Cisco debug command

Debug is a troubleshooting command that’s available from the privileged exec mode of Cisco IOS. It’s used to display information about various router operations and the related traffic generated or received by the router, plus any error messages.
It’s a useful and informative tool, but you really need to understand some important facts about its use. Debug is regarded as a very high-priority task because it can consume a huge amount of resources and the router forced to process-switch the packets being debugged. So you don’t just use debug as a monitoring tool – it’s meant to be used for short period of time and only as a troubleshooting tool. By using it, you can really find out some truly significant facts about both working and faulty software and/or hardware components.

Because debugging output takes priority over other network traffic, and because the debug all command generates more output than any other debug command, it can severely diminish the router’s performance – even render it unusable. So in virtually all cases, it’s best to use more-specific debug commands.