Cisco CCNA Exam Tutorial: Troubleshooting Directly Connected Serial Interfaces

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457

Summary:
Knowing how to configure and troubleshoot directly connected serial interfaces is a vital part of your CCNA studies rockawaii.com Learn the details from Chris Bryant, CCIE #12933.


Keywords:
Cisco, exam, dce, dte, clockrate, directly, connected, serial, interfaces, line, protocol, down, show, controller, Bryant, advantage, 12933


Article Body:
CCNA exam success depends largely on noticing the details, and this is especially true of configurations involving directly connected serial interfaces saintmarcarabians.com 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 scliew.com In the following example, R1 and R3 are directly connected at their Serial1 interfaces signgalonline.com 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 southernstyleproductions.com 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 starwinmedia.com In this example, though, we don't have physical access to the routers stevestonvillagegallery.com 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 stickerduderacing.com (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 studioyousb.com 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 thepreachersplatform.com If you see the combination shown below, the connection is physically fine but logically down titostouch.com That's generally the result of a needed keepalive not being present vegawebproductions.com 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 wmmfsg.com Know your show and debug commands and you're on your way to passing the CCNA!

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