Testing an Ethernet Cable from a Cisco Switch

An essential factor in troubleshooting LAN connectivity is by first ensuring that the physical connections are functional. When it comes to a Layer 2 network, this means the Switches, the Ports and the Ethernet cables that connect them all together.

For the Ethernet cable side of things, we usually have specialised tools which are able to give us detailed information about a cable when plugged in. Sometimes however, you may find yourself in a situation where you don’t have any tools to hand. Fear not! Though it isnt to be absolutely relied upon for a number of reasons, many Cisco Switches have the capability of conducting a cable test on a port by port basis.

This feature is called the Time-Domain Reflectometer (TDR) Test. Please note that not all Switches support this feature, so while handy, please do not rely on it in any shape or form. Think of it as an optional extra during troubleshooting that may save time if its supported on the Switch in question.

Lets see how we can use it.

Running the Test

To run the test, run the following command in enable mode on the Switch, specifying the interface you are interested in. Note that this will cause a temporary loss in connectivity on the port, only do this if that is ok.

SWITCH# test cable-diagnostics tdr interface [INTERFACE]

This step is as simple as that, but understand that we won’t see the results fill the screen instantly. It will take a short amount of time, and when the test is complete we will have to execute another command to see the results.

Viewing the Test Results

To view the results of the test, use the following command:

SWITCH# show cable-diagnostics tdr interface [INTERFACE]

You will example output like the below:

SWITCH# show cable-diagnostics tdr interface [INTERFACE]
TDR test last run on: May 14 16:54:12

Interface   Speed Local pair Pair length        Remote pair Pair status
---------   ----- ---------- ------------------ ----------- --------------------
[INTERFACE] 100M  Pair A     N/A                N/A         Normal
                  Pair B     N/A                N/A         Normal
                  Pair C     42   +/- 10 meters N/A         Normal
                  Pair D     N/A                N/A         Normal

Great! But how do we interpret this? Lets take a look.

Interpreting the Output

We can instantly see useful information like:

  • The Speed of the port (100Mbps)
  • The (rough) Length of the cable (42 meters, but also take into account the margin of error which is +/- 10 meters)
  • The Pair Status. This Refers to the individual wires within the Ethernet Cable (there are 4 pairs). As everything is ‘Normal’, it seems like everything is fine with the cable in this scenario.

But we can also have some other values for ‘Pair Status’. Lets have a look at what we might see.

Pair statusExplanation
NormalThis means that the Pair is working fine.
OpenThis means that the Pair does not form a closed circuit with the connection on either end.
In most cases, the cable needs either replacement or re-termination to resolve the issue.

There are more values that may pop up, but these are the ones that I have encountered so far. Essentially, if you see anything other than ‘Normal’, you have a problem!

I hope this has been helpful.

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