To pass the CCNA exam, you’ve got to master quite a few services and routing protocols that may be new to you. Between RIP, IGRP, EIGRP, OSPF, and switching, there are hundreds of details you’ve got to absorb! It’s easy to spend all your time on those topics and not pay proper attention to “easier” technologies, and then all of a sudden on exam day you can’t quite remember the details of those particular services.
One setup you’ve got to be more than familiar with is directly connecting serial interfaces on Cisco routers. This is also a valuable skill to have in your home lab, since it allows you to add segments to your network setup.
A Cisco serial interface is operating as a DTE by default. The problem is that when you take a cable and connect two routers directly by their serial interfaces (with a DTE/DCE cable, that is!), they’re both waiting for the other to send them a clock rate. One of the interfaces must act as the DCE and that interface must send the clock rate.
If you can see the DTE/DCE cable, you can tell by looking which router has the DCE interface connected to it – the letters...