Basic network troubleshooting is a vital part of your Cisco CCENT & CCNA exam studies, & it is obviously an important part of what we do as
network admins on a day-to-day basis. In today’s free Cisco certification training tutorial, we will take a look at the output of show interface serial under different
circumstances, concentrating on the physical & logical state of the interface.
To be an effective troubleshooter, you have to know how things look when
all is well, not just when something is broken! When an interface is functioning correctly, this is what we see at the top of the show interface output. I will use Serial0 for
all examples in this section.
Router1#show int serial0
Serial0 is up, line protocol is up
Of course, if the interface looked like that all the time,
we would not actually be troubleshooting, would we? 
Let us take a look at common show interface outputs that do indicate a problem, along with the solutions for these
issues.
Router1#show int serial0
Serial0 is administratively down, line protocol is down This one’s easy! administratively down means the interface is
indeed shut down. Open the interface with no shutdown.
Router1(config)#int serial0
Router1(config-if)#no shutdown
Always give the line
protocol a minute or so to come up after opening a serial interface.
Router1#show interface serial0
Serial0 is up, line protocol is up Success! Let us
look at another potential readout:
Router1#show interface serial 0
Serial0 is down, line protocol is down The physical interface is again down, but not
administratively down. This shows a physical problem, either with the interface or the cable attached to it… Or not attached, as the case may be.
From
years of experience, I can tell you that a cable that looks fully attached to a router may not be. If you see this specific output, check the cable & make certain it is
connected. Even if it looks like it is not, give it a little push in anyway. (Don’t push it through the other side of the router, that defeats the purpose of
troubleshooting!)
A lot of router cabinets are crowded, & if a cable is nudged accidentally by an admin’s elbow, that can be enough to bring the connection
down. (Not that I have ever done that, you understand!)
After making sure the cable is plugged in….
Router1#
20:14:47: %LINK-3-UPDOWN:
Interface Serial0, changed state to up
20:14:48: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0, changed state to up
… the link
comes back up. Of course, we will verify that before moving to the next example!
Router1#show int serial 0
Serial0 is up, line protocol is
up
There’s one more physical / line protocol combination we have to be ready to troubleshoot, & it is a common one:
Router1#show int serial
0
Serial0 is up, line protocol is down
Here, the interface is physically fine, but there is a logical problem. This can be due to an encapsulation
mismatch, such as when one partner in a point-to-point connection is configured for HDLC & the other for PPP.
Configuring both ends of the point-to-point
link for the same encapsulation will bring the line protocol back up.
Whether the DCE is a CSU/DSU or another Cisco router in a home lab, the DCE must
supply a clockrate to the DTE. If that clockrate isn’t present, the line protocol will come down.
The show interface serial command is vital for WAN
troubleshooting, & knowing how to handle the different results we have seen here are a major step forward in you passing your CCENT & CCNA exams! We’ll look
at some LAN troubleshooting techniques in the next installment of my exclusive Cisco CCENT & CCNA tutorial series!
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage, home of free
also visit his blog, which is updated many times daily with new Cisco certification articles, free tutorials, & daily CCNA / CCNP exam questions! For a FREE copy
of his latest e-books, “How To Pass The CCNA” & “How To Pass The CCNP”, just visit the website! You can also get FREE CCNA & CCNP exam questions
every day!
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