Network Step by Step: Troubleshoot Cisco routers and switches using the debug commands

Let’s take a look at a simple example. We are going to view RIP (Routing Information Protocol) in Debug mode.

Router# debug ip RIP

RIP protocol debugging is on

To verify what debugging is enabled, use this command:

Router# show debug

  RIP protocol debugging is on

The output from whatever type of debug is enabled will be sent to wherever the Cisco IOS logging system tells that output to go. Either you will receive the output on your screen, it will go to the buffered log on the router, or it will go to a syslog server across the network (or all of these).

To see what level the various outputs are set to and where the output will go, type:

Router# show logging

Syslog logging: enabled (1 messages dropped, 3 messages rate-limited,

                0 flushes, 0 overruns, xml disabled, filtering disabled)

    Console logging: level debugging, 8 messages logged, xml disabled,

                     filtering disabled

    Monitor logging: level debugging, 0 messages logged, xml disabled,

                     filtering disabled

    Buffer logging: level warnings, 2 messages logged, xml disabled,

                    filtering disabled

    Logging Exception size (4096 bytes)

    Count and timestamp logging messages: disabled

    Trap logging: level informational, 12 message lines logged

Log Buffer (51200 bytes):

*Jun  9 20:56:49.195: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up

*Jun  9 20:56:49.231: %LINK-3-UPDOWN: Interface FastEthernet0/1, changed state to up

Router#

The console should display RIP updates that are sent and received through the RIP protocol. Here is an example of what you might see for RIP debugging:

*Jun  9 21:13:56.471: RIP: sending v1 update to 255.255.255.255 via FastEthernet0/0 (1.1.1.1)

*Jun  9 21:13:56.471: RIP: build update entries - suppressing null update

*Jun  9 21:14:22.519: RIP: sending v1 update to 255.255.255.255 via FastEthernet0/0 (1.1.1.1)

*Jun  9 21:14:22.519: RIP: build update entries - suppressing null update

Remember that you should use Debug only for a short time to get a snippet of information, and then turn Debug off as it can be a serious performance hit on your router.

There are several commands for turning off Debug.

Router# no debug

If you type debug ?, you will see that there are over 200+ Debug commands, and each of those has many options. Debugging RIP is just a very simple example.