Troubleshoot Cisco routers and switches using the debug commands - Part 3

What are the three most common mistakes made when using Debug?

Using Debug can be a risky proposition, and even experienced admins have made mistakes when using it.

I’d say the number one common mistake is to forget that you have left Debug on in a production environment. Sometimes, we get so focused on resolving the issue that when we get it resolved, we are on to the next “opportunity” and forget to issue the no debug command to turn off debugging. I think that many a network admin can attest to horror stories of when they brought their router to its knees because they forgot this simple task of turning off Debug.

The second common mistake would be not realizing the effect on your router of issuing a lot of Debug commands at the same time. Remember that the router’s job is to forward packets, not to monitor processes and generate Debug messages. For example, you are having a problem with the packets on your router, so you issue the Debug statement debug ip packet. Then you decide that you want to view the events on the RIP protocol. Now, you have two separate Debug statements that are being processed and sent to the console. Debug statements are processed at a higher priority than other network traffic, so, needless to say, these Debug statements can jeopardize your router’s performance.

The third common mistake made with the Debug command is entering debug all or debug ip packet detail on a production router. Either one of these commands can crash a heavily loaded production router. Luckily, there is an “are you sure” prompt before these take effect; however, that hasn’t prevented every debug-related catastrophe. You should be as specific as possible when using Debug, and then turn it off as quickly as possible. Also, always test your Debug commands on a test router before using them in a production environment.