Thursday, October 26, 2017

Cable Testing using Cisco Switches

Everyday I learn something new. Today I learned that I can test layer 1 by using tools from the CLI of a switch. Some Cisco gear has a built in TDR which is pretty freaking cool!
  • A TDR (Time-Domain Reflectometer) is a tool used to locate faults in cables.

I stumbled upon a doc on cisco fourms that describes this feature in great detail but wanted to put some info on my blog as a reminder for myself.

Link to Cisco Post:



     test cable-diagnostics tdr interface [int]
     show cable-diagnostics tdr interface [int]

Example Output:
 Switch1# test cable-diagnostics tdr interface gi 1/0/1
 TDR test started on interface Gi4/30
 A TDR test can take a few seconds to run on an interface
 Use 'show cable-diagnostics tdr' to read the TDR results.

 Switch1# show cable-diagnostics tdr interface gi 1/0/1
 TDR test last run on: October 26 12:07:51
 Interface Speed Local pair Pair length        Remote pair Pair status
 --------- ----- ---------- ------------------ ----------- --------------------
 Gi1/0/1   1000M Pair A     58   +/- 10 meters Pair B      Normal
                 Pair B     58   +/- 10 meters Pair A      Normal
                 Pair C     58   +/- 10 meters Pair D      Normal
                 Pair D     58   +/- 10 meters Pair C      Normal

Reading the output: (I refered to the cisco site as posted above.)

     Date TDR was last ran on: October 26 12:07:51
     The interface tested: Gi1/0/1
     The speed that was negotiated: 1000M (Gigabit)
     Pair information: All Normal

Diagnosing the Output:

     What the pairs are used for:
          Pair A: Used for up/down status
          Pair B: FastEthernet (10/100)
          Pair C: Power over Ethernet
          Pair D: GigabitEthernet

     Pair Status Results:
          Normal: Good connection
          Open: No pin contact on one or more pairs
          Short: Short circuit
          Impedance Mismatched: Bad cable.
          Not Supported: (Most likely used for GigabitEthernet or PoE)
Additional Notes:
  • This was tested on a WS-C2960X-48FPD-L with 15.2(2)E6 code
  • The port never went down. I ran a constant ping to the PC. 
    • I would make sure that the port can be taken offline before doing a test!