Using Time Domain Reflectometry (TDR) to locate cable faults on EX switches

Submitted by Bart on Wed, 01/25/2012 - 21:16

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One of the little known features of the Juniper EX switches is Time domain reflectometry which can be used to locate cable faults.
This is especially useful for long cables or when a connection is made up of multiple patch cables, to figure out where the cable is damaged.

To start TDR on a port, issue the following command:

request diagnostics tdr start interface <interface>

The test will take a couple seconds. To see the results, use:

show diagnostics tdr interface <interface>

On a working link, the result looks something like this:

Interface TDR detail:
Interface name                  : ge-1/0/26
Test status                     : Passed
Link status                     :  UP
MDI pair                        : 1-2
  Cable status                  : Normal
  Distance fault                : 0 Meters
  Polartiy swap                 : Normal
  Skew time                     : 0 ns
MDI pair                        : 3-6
  Cable status                  : Normal
  Distance fault                : 0 Meters
  Polartiy swap                 : Normal
  Skew time                     : 0 ns
MDI pair                        : 4-5
  Cable status                  : Normal
  Distance fault                : 0 Meters
  Polartiy swap                 : Normal
  Skew time                     : 8 ns
MDI pair                        : 7-8
  Cable status                  : Normal
  Distance fault                : 0 Meters
  Polartiy swap                 : Normal
  Skew time                     : 0 ns
Channel pair                    : 1
  Pair swap                     : MDIX
Channel pair                    : 2
  Pair swap                     : MDIX
Downshift                       : No Downshift

When something is wrong with the cable, the results may vary of course. A cable which simply isn't connected to anything at the far end shows up like this:

Interface TDR detail:
Interface name                  : ge-1/0/16
Test status                     : Passed
Link status                     : Down
MDI pair                        : 1-2
  Cable status                  : Open
  Distance fault                : 52 Meters
  Polartiy swap                 : N/A
  Skew time                     : N/A
MDI pair                        : 3-6
  Cable status                  : Open
  Distance fault                : 50 Meters
  Polartiy swap                 : N/A
  Skew time                     : N/A
MDI pair                        : 4-5
  Cable status                  : Open
  Distance fault                : 50 Meters
  Polartiy swap                 : N/A
  Skew time                     : N/A
MDI pair                        : 7-8
  Cable status                  : Open
  Distance fault                : 52 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