How to (more) safely run debugs on Cisco routers

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From the computer of: abanks (41 recipes)
Created: Dec 15, 2006     Updated: Dec 18, 2006

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Running debug commands can bring a busy router to a crawl. Not sending the output to the console can help. This tech-recipe describes turning off console debugging output and techniques for viewing debug information in a router-friendly way.

Some debug commands, like

can cause a router to stop responding while it attempts to display all the data you've requested. Frequently, it's trying to force a lot of data down a 9600 baud (slow) console port.

So, we can turn this off!

But, now debugs are not very useful, since they won't display. We can telnet to the router, then

to send all the debug output down the pipe to the telnet client. Better, but this can still backfire.

We can just push all the messages to a buffer:

then view them with

Another option which will make some debug commands, like

safer is to specify an access-list for just the traffic you want. For example:

Will give you IP packet detail for any packets coming from or going to a host with ip address 1.1.1.1.