Wednesday, December 5, 2007

Using Cisco's Integrated TDR

Did you know most Cisco switches with 10/100/1000 RJ45 ports have TDR capabilities? Yes, it is true.

At the time of this writing it is my understanding that this feature is not supported on any 10/100 ports or SFP module ports.

So, what the TDR may help you with? Short answer: dealing with the physical problems outside of the switch. Long answer: it helps you detect certain types of twisted-pairs cabling problems. So you can troubleshoot:\
  • Open wires
  • Cut wires
  • Broken wires
  • Shorted wires

With the TDR functionallity you can find the length at which the twisted-pair wire is open.

Now, remember, since TDR is a port test, the port can will not handle any traffic for the duration of the test (generally, 1 minute). So, how do you run this feature? In two simple phases.

First run the diagnostic test and collect the results:

# test cable-diagnostics tdr interface GigabitEthernet 0/1

Second display the results:

# show cable-diagnostics tdr interface GigabitEthernet 0/1

Interface Speed Local pair Cable length Remote channel Status
Gi0/1 0Mbps 1-2 102 +-2m Unknown Fault
3-6 100 +-2m Unknown Fault
4-5 102 +-2m Unknown Fault
7-8 102 +-2m Unknown Fault

The previous command is set to be deprecated sometime in the future and will be replaced by:

# show diagnostic result interface GigabitEthernet 0/1

So you better know both. You never know when you might need it.


  1. Woah! I never knew Cisco switches have TDR capabilities. thanks for explaining the whole stuff now its crystal clear to me. Can u please update us more about cisco unidirectional link detection.

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