Recent Articles

Route Summarization And The OSPF Null Interface
CCNP exam success, particularly on the BSCI exam, demands you understand the details of route summarization.
Server Load Balancing (SLB)
When youŽre working on your BCMSN exam on your way to CCNP certification, youŽll read at length about how Cisco routers and multilayer switches can work to provide router redundancy - but thereŽs another helpful service, Server Load Balancing, that does the same for servers.

Microsoft to Release Speech Server Beta
On May 8, Microsoft announced availability of the beta version of Microsoft Speech Server 2007, which is scheduled to be released late this year...

Sun Sees Red, Sues Azul
Sun Microsystem's acquisition of Cobalt Networks in September 2000 has made it back into the headlines, as Sun files a lawsuit against former Cobalt and current Azul CEO Stephen DeWitt over theft of trade secrets...

U.S. Servers Favored by Hate Groups
Many of these group clips originate overseas, but are posted on sites hosted in America, because it's cheaper and easier to use a site hosted here.

Who Will Win The Switch Wars?
Why is it that people can look at a simple sentence - and interpret it in so many ways?...Now, how that can mean anything other than what it says is beyond me, but for those with diminished capacities, let me expand.
Can't Connect to Internet
We talked about DNS problems at Fallure to resolve, but there are other reasons why a particular machine may be unable to browse the Internet while others on the same lan still can.

Available from CrossTec, Activeworx Security Center includes new features for meeting SOX and HIPAA compliance requirements, improved tools for forensic investigation, and much more.

Troubleshooting Directly Connected Serial Interfaces

By Chris Bryant

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

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


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?

Available from CrossTec, Activeworx Security Center includes new features for meeting SOX and HIPAA compliance requirements, improved tools for forensic investigation, and much more.

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 and daily exam questions, as well as The Ultimate CCNA and CCNP Study Packages.

About NetworkNewz
NetworkNewz editors, writers and contributors focus on both the big picture and the details of networking. At NetworkNewz our goal is to deliver to you The Key To Network Management.

NetworkNewz is brought to you by:

-- NetworkNewzis an iEntry, Inc. publication --
iEntry, Inc. 2549 Richmond Rd. Lexington KY, 40509
2006 iEntry, Inc.  All Rights Reserved  Privacy Policy  Legal

archives | advertising info | news headlines | free newsletters | comments/feedback | submit article

">Unsubscribe from NetworkNewz.
To unsubscribe from NetworkNewz or any other iEntry publication, simply send an email request to:
The Keys To Network Management Ask Questions in the Networking Forum NetworkNewz News Archives About Us Feedback NetworkNewz Home Page About Archive News Downloads WebProWorld Forums Jayde iEntry Advertise Contact