Some Cisco switches have a way to check the condition of copper cables. This can be done via de command test cable-diagnostics tdr. TDR stands for Time Domain Reflector. More information about Time Domain Reflector can be found at the Cisco Support Community.

The command can be very useful for basic layer 1 troubleshooting.

core01#test cable-diagnostics tdr int gig 5/0/21
TDR test started on interface Gi5/0/21
A TDR test can take a few seconds to run on an interface
Use ‘show cable-diagnostics tdr’ to read the TDR results.
core01#show cabl
core01#show cable-diagnostics tdr int gig 5/0/21
TDR test last run on: August 26 10:10:28

Interface Speed Local pair Pair length        Remote pair Pair status
——— —– ———- —————— ———– ——————–
Gi5/0/21  1000M Pair A     5    +/- 10 meters Pair A      Normal
Pair B     4    +/- 10 meters Pair B      Normal
Pair C     4    +/- 10 meters Pair C      Normal
Pair D     4    +/- 10 meters Pair D      Normal

The output above shows a successful measurement. The next example shows a failing cable-diagnostics test.

core01#test cable-diagnostics tdr interface gigabitEthernet 5/0/22
Link state may be affected during TDR test
TDR test started on interface Gi5/0/22
A TDR test can take a few seconds to run on an interface
Use ‘show cable-diagnostics tdr’ to read the TDR results.
core01#show cable-diagnostics tdr interface gig 5/0/22
TDR test last run on: August 26 10:04:43

Interface Speed Local pair Pair length        Remote pair Pair status
——— —– ———- —————— ———– ——————–
Gi5/0/22  100M  Pair A     N/A                Pair B      Normal
Pair B     N/A                Pair A      Fail
Pair C     N/A                Pair D      Normal
Pair D     N/A                Pair C      Normal

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René Jorissen

Co-owner and Solution Specialist at 4IP Solutions

René Jorissen works as Solution Specialist for 4IP in the Netherlands. Network Infrastructures are the primary focus. René works with equipment of multiple vendors, like Cisco, Aruba Networks, FortiNet, HP Networking, Juniper Networks, RSA SecurID, AeroHive, Microsoft and many more. René is Aruba Certified Edge Expert (ACEX #26), Aruba Certified Mobility Expert (ACMX #438), Aruba Certified ClearPass Expert (ACCX #725), Aruba Certified Design Expert (ACDX #760), CCNP R&S, FCNSP and Certified Ethical Hacker (CEF) certified. You can follow René on Twitter and LinkedIn.

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