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Cisco CCNA Exam Tutorial: Directly Connected Serial Interfaces

To pass the CCNA exam, you've got toHD unit 1, idb = 0x1DBFEC, driver
master quite a few services and routingstructure at 0x1E35D0buffer size 1524 HD
protocols that may be new to you.unit 1, V.35 DTE cable
Between RIP, IGRP, EIGRP, OSPF, andI left off the 16 or so rows of
switching, there are hundreds of detailsinformation that comes after this, but
you've got to absorb! It's easy to spendthis is the information we need right
all your time on those topics and notnow. If R1's got the DTE cable end, the
pay proper attention to "easier"other router should have the DCE end:
technologies, and then all of a suddenR3#show controller serial 1
on exam day you can't quite remember theHD unit 1, idb = 0x1C44E8, driver
details of those particular services.structure at 0x1CBAC8buffer size 1524 HD
One setup you've got to be more thanunit 1, V.35 DCE cable
familiar with is directly connectingWe know now that R3 needs to supply a
serial interfaces on Cisco routers. Thisclock rate to R1. There's a hint of a
is also a valuable skill to have in yourproblem in just that little bit of
home lab, since it allows you to addcommand output - do you see what it is?
segments to your network setup.Let's run show interface serial1 to get
A Cisco serial interface is operating asmore information.
a DTE by default. The problem is thatR3#show int s1
when you take a cable and connect twoSerial1 is up, line protocol is down
routers directly by their serialThe line protocol is down because there
interfaces (with a DTE/DCE cable, thatis no clockrate being supplied by R3. If
is!), they're both waiting for the otherthere has been, we would have seen that
to send them a clock rate. One of thein the output of show controllers serial
interfaces must act as the DCE and that1.
interface must send the clock rate.This is simple enough to fix, though!
If you can see the DTE/DCE cable, youWe'll use the command clockrate 56000 on
can tell by looking which router has theR3's serial1 interface, and the line
DCE interface connected to it - theprotocol will soon come up.
letters "DTE" or "DCE" will either beR3(config)#int s1
molded into the connector itself, or ifR3(config-if)#clockrate 56000
it's an older cable there should be a1w2d: %LINEPROTO-5-UPDOWN: Line protocol
little piece of tape on the cable thaton Interface Serial1, changed state to
tells you what the interface type is.up
But what if you have no access to theThis is a simple concept, but there are
cable, or there are other cables alla few details you must keep in mind! For
around it and you can't see what type ita home lab configuration, you'll need a
is?DTE/DCE cable to make this work. If you
Run the command "show controller serialcannot see the cable connectors, run
x", with x representing the interfaceshow controllers serial x to see if the
number the cable's connected to. Thererouter has the DTE or DCE end of the
will be quite a bit of output from thiscable attached. On the interface with
command, but the information you need isthe DCE attached, use the clockrate
right at the top:command to bring the line protocol up.
R1#show controller serial 1It's just that simple!



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