How to troubleshoot cabling and layer 1 ethernet issues with Cisco switches

Ever had a Cisco PoE phone or a wifi access point reboot spontaneously? How would you know if there was bad cabling work done? Or more precisely, where was bad patch? How can you troubleshoot cabling problems using a Cisco switch?

Introducing the Time Domain Reflector (TDR) feature in Cisco switches. You can use the TDR feature to diagnose and troubleshoot cabling problems. TDR is supported only on copper Ethernet 10/100/1000 ports. It is not supported on 10/100 ports or small form-factor pluggable (SFP) module ports.

Here’s what I ran to determine there was a cabling issue with one of the drops connecting to the phone.

switch#test cable-diagnostics tdr interface gigabitEthernet 1/0/42
TDR test started on interface Gi1/0/42
A TDR test can take a few seconds to run on an interface
Use 'show cable-diagnostics tdr' to read the TDR results.
switch#sh cable-diagnostics tdr interface gigabitEthernet 1/0/42
TDR test last run on: May 08 13:30:55

Interface Speed Local pair Pair length        Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Gi1/0/42  100M  Pair A     73   +/- 0  meters N/A         Normal
                Pair B     73   +/- 0  meters N/A         Normal
                Pair C     73   +/- 1  meters N/A         Short/Crosstalk
                Pair D     73   +/- 1  meters N/A         Short/Crosstalk

No wonder the phone has been resetting by itself!

As you can see in the output. Pairs C & D have been shorted. This affects delivery of Power (pair C) and Gigabit operation, although not used.

The following link that gives a much better and detail explanation of how to use TDR and when to use it.

Great read indeed.

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About the Author

Alfred Tong
Author and owner of this blog. A Networking enthusiast, full time networking and systems Engineer. Generally curious about all things IT. Certifications: GIAC GSEC, CCNP-S, CCNP, CCSP, CCDP, CCNA, RHCE, JNCIA - FWV