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Diagnosing 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.
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.
The TDR test is supported on the following switches and interfaces:
- EX2200, EX3200, EX3300, and EX4200 switches—RJ-45 network interfaces. The TDR test is not supported on management interfaces and SFP interfaces.
- EX6200 and EX8200 switches—RJ-45 network interfaces on line cards.
 | Note: We recommend running the TDR test on an interface when there is no traffic on the interface. |
To diagnose a cable problem by running the TDR test:
- Run the request diagnostics tdr command.
user@switch> request diagnostics tdr start interface ge-0/0/10 Interface TDR detail: Test status : Test successfully executed ge-0/0/10
- View the results of the TDR test with the show diagnostics tdr command.
user@switch> show diagnostics tdr interface ge-0/0/10 Interface TDR detail: Interface name : ge-0/0/10 Test status : Passed Link status : Down MDI pair : 1-2 Cable status : Normal Distance fault : 0 Meters Polartiy swap : N/A Skew time : N/A MDI pair : 3-6 Cable status : Normal Distance fault : 0 Meters Polartiy swap : N/A Skew time : N/A MDI pair : 4-5 Cable status : Open Distance fault : 1 Meters Polartiy swap : N/A Skew time : N/A MDI pair : 7-8 Cable status : Normal Distance fault : 0 Meters Polartiy swap : N/A Skew time : N/A Channel pair : 1 Pair swap : N/A Channel pair : 2 Pair swap : N/A Downshift : N/A
- 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.
| Note: 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:
user@switch> show diagnostics tdr interface ge-0/0/22 Interface TDR detail: Interface name : ge-0/0/22 Test status : Started
- 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:
user@switch> show diagnostics tdr Interface Test status Link status Cable status Max distance fault ge-0/0/0 Passed UP OK 0 ge-0/0/1 Not Started N/A N/A N/A ge-0/0/2 Passed UP OK 0 ge-0/0/3 Not Started N/A N/A N/A ge-0/0/4 Passed UP OK 0 ge-0/0/5 Passed UP OK 0 ge-0/0/6 Passed UP OK 0 ge-0/0/7 Not Started N/A N/A N/A ge-0/0/8 Passed Down OK 0 ge-0/0/9 Not Started N/A N/A N/A ge-0/0/10 Passed Down Fault 1 ge-0/0/11 Passed UP OK 0 ge-0/0/12 Not Started N/A N/A N/A ge-0/0/13 Not Started N/A N/A N/A ge-0/0/14 Not Started N/A N/A N/A ge-0/0/15 Not Started N/A N/A N/A ge-0/0/16 Not Started N/A N/A N/A ge-0/0/17 Not Started N/A N/A N/A ge-0/0/18 Not Started N/A N/A N/A ge-0/0/19 Passed Down OK 0 ge-0/0/20 Not Started N/A N/A N/A ge-0/0/21 Not Started N/A N/A N/A ge-0/0/22 Passed UP OK 0 ge-0/0/23 Not Started N/A N/A N/A
