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Cisco CCENT / CCNA Certification Exam Training :

Troubleshooting Physical Interfaces And Line Protocols

By Chris Bryant, CCIE #12933

Now with a bonus video from my CCNA YouTube Video Channel!


Basic network troubleshooting is a vital part of your Cisco CCENT and CCNA exam studies, and it's 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'll take a look at the output of show interface serial under different circumstances, concentrating on the physical and 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'll 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 wouldn't be troubleshooting, would we? :)  Let's 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

We'll pick up with this tutorial right after this important message!

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Now back to the tutorial!

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's 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 indicates 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 particular output, check the cable and make sure it's connected.  Even if it looks like it's 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, and if a cable is nudged accidentally by an admin's elbow, that can be enough to bring the connection down. (Not that I've ever done that, you understand!)

After making sure the cable is plugged in....

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'll 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, and it's 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 and the other for PPP.

Encapsulation Mismatch Brings Line Protocol Down

Configuring both ends of the point-to-point link for the same encapsulation will bring the line protocol back up.

Encapsulation Match, Line Protocol Comes 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 is not present, the line protocol will come down.

DCE Gives Clockrate To DTE


The show interface serial command is vital for WAN troubleshooting, and knowing how to handle the different results we've seen here are a major step forward in you passing your CCENT and CCNA exams! We’ll look at some LAN troubleshooting techniques in the next installment of my exclusive Cisco CCENT / CCNA certification exam tutorial series!

You'll enjoy this tutorial and video on the OSI Networking Model, too!

Chris B.