Diagnose a Faulty Twisted-Pair Cable (CLI Procedure)

Problem

A 10/100/1000BASE-T Ethernet interface has connectivity problems that you suspect might be caused by a faulty cable.

Cause

Ethernet cables can become faulty or degrade due to physical damage, poor termination, electromagnetic interference, or wear over time. Issues such as open circuits, short circuits, impedance mismatches, pair swaps, polarity reversals, or excessive pair skew can disrupt proper signal transmission which leads to intermittent failed connectivity on the interface.

Solution

Use the time domain reflectometry (TDR) test to determine whether a twisted-pair Ethernet cable is faulty.

The TDR test:

• Detects and reports faults for each twisted pair in an Ethernet cable. Faults detected include open circuits, short circuits, and impedance mismatches.

• Reports the distance to fault to within 1 meter.

• Detects and reports pair swaps, pair polarity reversals, and excessive pair skew.

Use Feature Explorer to confirm platform and release support for specific features.

Review the Platform-Specific Time Domain Reflectometry (TDR) Behavior section for notes related to your platform.

To diagnose a cable problem by running the TDR test:

1. Run the request diagnostics tdr command.

2. View the results of the TDR test with the show diagnostics tdr command.

3. Examine the Cable status field for the four MDI pairs to determine if the cable has a fault. In the preceding example, the twisted pair on pins 4 and 5 is broken or cut at approximately one meter from the ge-0/0/10 port connection.

   The Test Status field indicates the status of the TDR test, not the cable. The value Passed means the test completed—it does not mean that the cable has no faults.

The following is additional information about the TDR test:

• The TDR test can take some seconds to complete. If the test is still running when you execute the show diagnostics tdr command, the Test status field displays Started. For example:

• You can terminate a running TDR test before it completes by using the request diagnostics tdr abort interface interface-name command. The test terminates with no results, and the results from any previous test are cleared.

• You can display summary information about the last TDR test results for all interfaces on the switch that support the TDR test by not specifying an interface name with the show diagnostics tdr command. For example: