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Cisco CCNA Exam Tutorial: Troubleshooting Directly Connected Serial Interfaces |
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CCNA exam success depends largely on noticing the details,
and this is especially true of configurations involving directly
connected serial interfaces. And of course, it's not enough
to notice these details - you've got to know what to do about
them!
A Cisco router is a DTE by default, but directly connecting
two DTEs with a DCE/DTE cable is not enough. In the following
example, R1 and R3 are directly connected at their Serial1
interfaces. The line goes up briefly after being opened, but
the line protocol goes down after about 30 seconds.
R3(config-if)#int s1
R3(config-if)#ip address 172.12.13.3 255.255.255.0
R3(config-if)#no shutdown
2d18h: %LINK-3-UPDOWN: Interface Serial1, changed state to
up
2d18h: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1,
changed state to up
R3(config-if)#
2d18h: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1,
changed state to down
The problem is that one of the routers needs to act as the
DCE in order for the line protocol to come up and stay up.
If this were your CCNA / CCNP home lab, you could just go
over and look at the DTE/DCE cable to see which router had
the DCE end of the cable attached. In this example, though,
we don't have physical access to the routers. How can we tell
which router has the DCE end of the cable attached?
R3#show controller serial 1
HD unit 1, idb = 0x1C44E8, driver structure at 0x1CBAC8
buffer size 1524 HD unit 1, V.35 DCE cable
The show controller command gives us this information. (There's
a lot more output that this with this command, but it's unimportant
for our purposes.) The router with the DCE end of the cable
needs to supply a clock rate to the DTE, and we'll do just
that with the interface-level clockrate command.
R3#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R3(config)#int serial1
R3(config-if)#clockrate 56000
2d18h: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1,
changed state to up
In just a few seconds, the line protocol goes up and stays
up.
When troubleshooting a connection, always run show interface
first. If you see the combination shown below, the connection
is physically fine but logically down. That's generally the
result of a needed keepalive not being present. With Frame
Relay, it's probably an LMI issue, but with directly connected
serial interfaces the issue is most likely the DCE end of
the connection not supplying clockrate.
R3#show interface serial 1
Serial1 is up, line protocol is down
Troubleshooting is a big part of the job, and it's a big
part of the Cisco CCNA and CCNP programs as well. Know your
show and debug commands and you're on your way to passing
the CCNA!
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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