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Cisco CCNA Exam Tutorial Directly Connected Serial Interfaces |
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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
"DTE" or "DCE" will either be molded into
the connector itself, or if it's an older cable there should
be a little piece of tape on the cable that tells you what
the interface type is. But what if you have no access to the
cable, or there are other cables all around it and you can't
see what type it is?
Run the command "show controller serial x", with
x representing the interface number the cable's connected
to. There will be quite a bit of output from this command,
but the information you need is right at the top:
R1#show controller serial 1
HD unit 1, idb = 0x1DBFEC, driver structure at 0x1E35D0
buffer size 1524 HD unit 1, V.35 DTE cable
I left off the 16 or so rows of information that comes after
this, but this is the information we need right now. If R1's
got the DTE cable end, the other router should have the DCE
end:
R3#show controller serial 1
HD unit 1, idb = 0x1C44E8, driver structure at 0x1CBAC8
buffer size 1524 HD unit 1, V.35 DCE cable
We know now that R3 needs to supply a clock rate to R1. There's
a hint of a problem in just that little bit of command output
- do you see what it is? Let's run show interface serial1
to get more information.
R3#show int s1
Serial1 is up, line protocol is down
The line protocol is down because there is no clockrate being
supplied by R3. If there has been, we would have seen that
in the output of show controllers serial 1.
This is simple enough to fix, though! We'll use the command
clockrate 56000 on R3's serial1 interface, and the line protocol
will soon come up.
R3(config)#int s1
R3(config-if)#clockrate 56000
1w2d: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1,
changed state to up
This is a simple concept, but there are a few details you
must keep in mind! For a home lab configuration, you'll need
a DTE/DCE cable to make this work. If you cannot see the cable
connectors, run show controllers serial x to see if the router
has the DTE or DCE end of the cable attached. On the interface
with the DCE attached, use the clockrate command to bring
the line protocol up. It's just that simple!
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage,
home of free CCNA and CCNP tutorials, The Ultimate CCNA Study
Package, and Ultimate CCNP Study Packages.
For a FREE copy of his latest e-books, “How To Pass
The CCNA” and “How To Pass The CCNP”, visit
the website and download your free copies. You can also get
FREE CCNA and CCNP exam questions every day! Pass the CCNA
exam with The Bryant Advantage!
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