debug command

The debug command displays information about the Cisco device operations, generated or received traffic, and any error messages. The information are provided in real-time until the user disables debugging or the device is restarted.
The debug operation takes a lot of CPU resources and should not be used often in production environments. It is meant to be used as a troubleshooting tool for only a short period of time. You can choose to debug only specific events – for example, EIGRP information, received ICMP messages, etc.

Consider the following example:

R1#debug ip icmp
ICMP packet debugging is on
ICMP: echo reply sent, src, dst

ICMP: echo reply sent, src, dst

In the example above you can see that I’ve enabled debugging only for the ICMP events (such as pings). In the output you can see that R1 has responded to the device with the IP address of with two ping reply packets.

To disable debugging of the ICMP events, simply re-enter the command with the no keyword in front of it:

R1#no debug ip icmp
ICMP packet debugging is off

To debug only RIP messages, we would run the following command:

R1#debug ip rip
RIP protocol debugging is on
R1#RIP: received v2 update from on GigabitEthernet0/0 via in 2 hops via in 1 hops
RIP: received v2 update from on GigabitEthernet0/0 via in 16 hops via in 16 hops

You can enable debugging of everything happening on your device by issuing the debug all command (do not use this command on production devices, since it can produce a lot of output and crash the device!):

debug all command example

To disable this command, type the undebug all command (or the un all shortcut).

Prerequisites for 200-301

200-301 is a single exam, consisting of about 120 questions. It covers a wide range of topics, such as routing and switching, security, wireless networking, and even some programming concepts. As with other Cisco certifications, you can take it at any of the Pearson VUE certification centers.

The recommended training program that can be taken at a Cisco academy is called Implementing and Administering Cisco Solutions (CCNA). The successful completion of a training course will get you a training badge.

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