Diagnosing Serial Link Symptoms with the show interface Command


Diagnosing Serial Link Symptoms with the show interface Command

There are many different types of encapsulations and protocols that can run over serial links. This can create many possible configuration problems. Most serial link problems unrelated to hardware are caused by a mismatched configuration on either side of the serial link. Both ends of a serial link must use the same encapsulation types, speed, and protocol types.

You can use the show interface command to reveal information that will help you in troubleshooting serial links. The output of the show interface command gives you a variety of information about each interface's status.

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For the exam, you should know that encapsulation is a technique used by layered protocols to add header information to a protocol data unit (PDU). This process sometimes adds a footer that includes a Cyclic Redundancy Check (CRC).


Each of the following sub-sections discusses a problem that can be revealed in the output of the show interface command, and discusses some potential causes for those problems. Here are the serial interface symptoms diagnosed from the show interface command in these sections:

  • Interface is administratively down; line protocol is down

  • Interface is down; line protocol is down

  • Keepalive sequencing not incrementing

  • Interface is up; line protocol is down

  • Interface is up; line protocol is up (looped)

  • Incrementing carrier transition counter

  • Incrementing interface resets

  • Input drops , CRC, and framing errors

Interface Is Administratively Down; Line Protocol Is Down

The Interface Is Administratively Down; Line Protocol Is Down error shown by the router indicates that a manual change to the router's configuration has been made.

Here are some potential causes:

  1. The interface has not been placed in shutdown state by an administrator.

  2. Check to see that a duplicate IP address has not been used.

Interface Is Down; Line Protocol Is Down

The Interface Is Down; Line Protocol Is Down error shown by the router indicates that the line is not making a connection and the protocols configured are not sending valid keepalive messages. This can be an indicator of many types of problems.

To troubleshoot this problem, use the following steps:

  1. Check the cabling to make sure there is not a break or other problem affecting connectivity.

  2. Check that a carrier signal from a local provider exists.

  3. Consider a hardware failure on the interface or CSU/DSU.

Interface Is Up; Line Protocol Is Down

To troubleshoot this problem, use the following steps:

  1. Check the interface for a problem with addressing, protocol, or interface configurations.

  2. Check with the local provider to verify that it is not having line issues.

Keepalive Sequencing Not Incrementing

The Keepalive Sequencing Not Incrementing error is usually an indicator of a line problem, meaning that the configured protocols and the router are not receiving keepalive messages.

To troubleshoot this problem, use the following steps:

  1. Check for a local hardware failure.

  2. Check the remote hardware for configuration or hardware failures.

  3. Check whether the line is noisy or has other connection problems. You may have to contact your service provider to test this.

  4. Check for a timing mismatch.

Interface Is Up; Line Protocol Is Up (Looped)

To troubleshoot this problem, use the following step:

  1. Verify whether the circuit is in loopback. In serial lines and telephone systems, a loopback is a test signal sent to a network destination that is returned as received to the originator. The returned signal may help diagnose a problem. Sending a loopback test to each telephone system piece of equipment in succession, one at a time, is a technique for isolating a problem.

Incrementing Carrier Transition Counter

An increasing Carrier Transition Counter on a serial interface shown by the show interface command output indicates that the interface is not staying active.

To troubleshoot this problem, use the following steps:

  1. Check for cable faults.

  2. Check for line issues with the local provider.

  3. Consider a hardware failure on the interface or Channel Service Unit/Digital Service Unit (CSU/DSU).

Incrementing Interface Resets

When the Interface Resets counter is incrementing, this means the interface is performing the equivalent of reboot.

To troubleshoot this problem, use the following steps:

  1. Check for physical cabling problems or faults causing the loss of the Carrier Detect.

  2. Consider a hardware failure on the interface, router, or the CSU/DSU.

  3. Check for congestion or overuse of the bandwidth on the line.

Input Drops, CRC, and Framing Errors

Increasing line drops, CRC, and framing errors can be an indication of a serious problem. When these errors occur, sent data is not making it to its destination.

To troubleshoot this problem, use the following steps:

  1. Check for mismatched line speeds.

  2. Check with the local provider for issues.

  3. Check whether the line is noisy.

  4. Check for faulty cabling.

  5. Consider a hardware failure on the interface or CSU/DSU.

  6. Check that the line speed does not overload the router's capacity.



CCNP CIT Exam Cram 2 (642-831)
CCNP CIT Exam Cram 2 (Exam Cram 642-831)
ISBN: 0789730219
EAN: 2147483647
Year: 2003
Pages: 213
Authors: Sean Odom

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