Debugging

Debugging may be new to you. It is available only on IOS-based switches, and there is no comparable
feature in CatOS. Of course, debugging has been inside routers since time began, so
those of you familiar with router IOS already know something about it. For those wanting to
learn the complete story of debugging, I refer you to CCNP: Cisco Internetwork Troubleshooting
Study Guide, by Arthur Pfund and Todd Lammle (Sybex, 2004).
Debugging is the process whereby you can gather information about specific activities going on
in the switch as they happen. Bearing in mind that debugging commands often have several extensions
allowing greater granularity of capture, you must remember that the context-sensitive help
provides the best guide to what debugging commands you can use.

Debugging Danger!
Not too long ago, I was consulting for a large ISP, and we were working as a team making lots
of changes to customer networks in the wee small hours of the morning. At one stage, one of
the guys needed to debug some activity on the customer router, and he was a little worried
about the effect. Because we had no time to run tests on the debug, I suggested that he set a
reload timer on the router in question so that it would reboot in five minutes if everything went
wrong. Well, things started off fine, but when he typed the undebug all command, he got a little
confused and typed debug all instead.
The target router lasted about 30 seconds before it terminated his Telnet session and overloaded
the memory and processor. Fortunately, it reloaded about two minutes later, and all
was well. He bought the beers. The moral of this story is don’t ever use the debug all command

Debugging is not free. Debugging takes place in the router processor at the heart of the
switch, and uses system buffers to store debugging information. If you try to debug too much
all at once, then you run the genuine risk of preventing the switch from functioning due to an
overworked processor and overloaded memory. Debugging should therefore be used like a surgeon’s
scalpel, cutting finely into what you need to see. Don’t use debugging like a club!
It is easy to forget precisely which debugging command you have entered, and therefore commands
exist to disable all debugging activity. There are two choices; no debug all and
undebug all work equally well.

Terry_2950#no debug all
All possible debugging has been turned off
Terry_2950#undebug all
All possible debugging has been turned off
Terry_2950#
outside the lab or classroom!