109 - 238 CCNA 2: Routers and Routing Basics v3.1 Instructor Guide – Module 9 Copyright © 2004, Cisco Systems, Inc.
the troubleshooting process. Students need to realize that there is a lot of output that will be
completely unfamiliar to them. The two main reasons for them to use this command are to
discover the cable type connected to the serial interface and to see the clock rate on the
interface with the DCE cable.
9.3.7 Introduction to debug The debug command is used to display dynamic data and events. The debug command
output gives more insight into the current events of the router. The dynamic output of the
debug command has a high performance cost. It produces a high processor overhead that
disrupts normal router operation. Debug should be used conservatively. Stress to the students
that debug is a very important tool. However, this command can disrupt router operations and
cause network performance to decrease drastically. It should only be used to diagnose a
problem and then it should be turned off.