Cable Test on Cisco Catalyst Switches

Sometimes an ethernet cable attached to a Cisco Catalyst switch might need to have simple testing done, but there is no technician on site and no testing hardware close by. In these instances, it is possible to run a simple TDR test from the switchport the cable is connected to.

The process is simple, but first, a few things to note:

• FastEthernet switch ports can only test the 2 pairs used for FastEthernet; if it is a PoE port and you are attempting to troubleshoot issues with the power pairs, the cable may need to be relocated temporarily to an available GigabitEthernet port so that all 4 pairs may be tested.
• If there is a clean break in the cable (all 4 pairs) and the initial length of the cable is unknown, the test may not indicate that something is wrong but it could point to the location of the break.
• Nothing can be connected to the other end of the cable for the test to work.

A brief overview of the process:

1. Determine the switchport the cable is connected to.
2. Ensure no other device is connected to the far end of the cable.
3. From privileged exec mode, issue the 'test cable tdr interface <int>' command.
4. Wait a few seconds for the test to complete, then issue the 'show cable tdr interface <int>' command to view the results.

Also note that in the tests I have abbreviated in some places per the running IOS on the switches (IOS 15 in all cases) where 'test cable-diagnostics tdr interface' may be abbreviated 'test cable tdr int' or possibly abbreviated more.


3 examples of the test:

1. FastEthernet interface, nothing abnormal:

switch#test cable tdr int f0/6
TDR test started on interface Fa0/6
A TDR test can take a few seconds to run on an interface
Use 'show cable-diagnostics tdr' to read the TDR results.
switch#show cable tdr int f0/6
TDR test last run on: October 18 14:15:32

Interface Speed Local pair Pair length        Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Fa0/6     auto  Pair A     34   +/- 1  meters N/A         Open
                Pair B     34   +/- 1  meters N/A         Open
                Pair C     N/A                N/A         Not Supported
                Pair D     N/A                N/A         Not Supported
switch#



2. FastEthernet interface with a break, the first test had the device connected and the second test the device on the far end of the cable was disconnected:

switch#test cable tdr int f0/13                 ! TEST 1
Link state may be affected during TDR test
TDR test started on interface Fa0/13
A TDR test can take a few seconds to run on an interface
Use 'show cable-diagnostics tdr' to read the TDR results.
switch#show cable-diagnostics tdr int f0/13
TDR test last run on: March 13 16:52:58

Interface Speed Local pair Pair length        Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Fa0/13    10M   Pair A     N/A                Pair B      Not Supported
                Pair B     N/A                Pair A      Not Supported
                Pair C     N/A                Pair D      Not Supported
                Pair D     N/A                Pair C      Not Supported

switch#


switch#test cable tdr int f0/13                 ! TEST 2
TDR test started on interface Fa0/13
A TDR test can take a few seconds to run on an interface
Use 'show cable-diagnostics tdr' to read the TDR results.
switch#show cable-diagnostics tdr int f0/13
TDR test last run on: March 13 16:54:52

Interface Speed Local pair Pair length        Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Fa0/13    auto  Pair A     5    +/- 1  meters N/A         Open
                Pair B     59   +/- 1  meters N/A         Open
                Pair C     N/A                N/A         Not Supported
                Pair D     N/A                N/A         Not Supported
switch#



3. GigabitEthernet interface with a short, also showing all 4 pairs being tested:

switch#test cable tdr int g0/2
TDR test started on interface Gi0/2
A TDR test can take a few seconds to run on an interface
Use 'show cable-diagnostics tdr' to read the TDR results.
switch#show cable tdr int g0/2
TDR test last run on: October 17 21:16:47

Interface Speed Local pair Pair length        Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Gi0/2     auto  Pair A     56   +/- 2  meters N/A         Open
                Pair B     6    +/- 2  meters N/A         Short
                Pair C     58   +/- 2  meters N/A         Open
                Pair D     56   +/- 2  meters N/A         Open
switch#

On the last test, keep in mind the +/- 2 meters as the error deviation and that the 58-meter pair is within the range of the 56-meter pairs. Obviously in this case, the 6-meter pair showing a short is the one that stands out. If the short is toward the end of the cable or on a patch cable or punchdown on the far end of the cable it may be difficult to narrow down those results.

Another catch: if the switch you are using has gigabit ports that may be selected between copper and sfp, the port will not auto-detect that the copper port is being used since the cable will be open for the test. You can set this manually from configuration mode:

switch(config)# int g0/2
switch(config)# media-type ?
  auto-select  Use whichever connector is attached
  rj45         Use RJ45 connector
  sfp          Use SFP connector
switch(config)# media-type rj45
switch(config)# exit
switch#

Don't forget to set it back to 'media-type auto-select' when you are done (and definitely do not write the change if it is not part of your standard config, but in that case it should already be set).


Hopefully this has been informative and contributes to another tool in the network troubleshooting toolbox!