CISCO IOS in a Nutshell

Debugging

Almost every aspect of the router’s configuration can have debugging enabled. For example, we could say:

debug chat

With debugging on, any chat script activity is logged to the router’s console (or to the syslog server, if configured). To disable logging, use the undebug command:

undebug chat

If you forget what debugging you turned on, use the undebug all command to turn all debugging off:

undebug all

There is a debug all command, but—unless you have major problems—there is rarely a good need for it. When it is executed, all debug information possible is flooded to your screen. You’ll be overwhelmed with data, even if logging is enabled. The best practice is to activate debugging only for the items in which you’re interested.

Using Debugging in Practice

The debug command is an extremely powerful troubleshooting tool. Assume that you have a serial interface on a router named Baltimore. At the other end of this interface is a router named New-York with an IP address of 10.1.2.55. You are having trouble communicating with the New-York router. One strategy for analyzing the problem is to enable IP-packet debugging on the Baltimore router to see if packets from New-York are arriving. To do so, use the command debug ip packet:

baltimore#debug ip packet IP packet debugging is on baltimore# IP: s=10.1.2.55 (Serial0), d=255.255.255.255, len 72, rcvd 2 IP: s=10.1.2.29 (Serial1), d=255.255.255.255, len 72, rcvd 2 IP: s=10.1.2.97 (local), d=255.255.255.255 (Ethernet0), len 72, sending ...