How to Read Cable TDR Testing Result

[PolarPanda]

Hi network gurus, 

           I have a hard time to determine which part cable is broken at below. Do both short and open mean that we have cable issue from patch panel to the Ethernet port on the wall (the cable run inside the wall/ceiling), and the cable from switch port to patch panel is normal? Thank you. 

Swtich01#
Interface Speed Local pair Pair length        Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Gi2/0/3  100M  Pair A     0    +/- 1  meters Pair A      Normal
                Pair B     0    +/- 1  meters Pair B      Normal
                Pair C     47   +/- 1  meters N/A         Short
                Pair D     49   +/- 1  meters N/A         Short

Switch 02#

Interface Speed Local pair Pair length        Remote pair Pair status

--------- ----- ---------- ------------------ ----------- --------------------

Gi1/0/20  10M   Pair A     0    +/- 1  meters Pair B      Normal

                Pair B     0    +/- 1  meters Pair A      Normal

                Pair C     22   +/- 1  meters N/A         Open

                Pair D     23   +/- 1  meters N/A         Open

[Leo Laohoo]

Is the TDR result taken from a PoE switch?

[royce.varughese.1984]

Yes it is

[Leo Laohoo]

Turn off PoE before doing a TDR.  

conf t
interface Tw1/0/21
 power inline NEVER
 do test cable tdr interface Tw1/0/21
<WAIT FOR ABOUT 10 SECONDS>
 do show cable tdr interface Tw1/0/21
<ONCE THE TEST IS COMPLETED TURN ON POE>
 power inline auto
end

The reason for turning off PoE when conducting TDR is because of known bug with the TDR result affecting exclusively IOS-XE (Bug IDs above).

If PoE is not turned off prior to conducting a TDR, the results will be wildly inaccurate and unreliable.

[royce.varughese.1984]

Interesting. When i turn POE off the port, interface goes down. Even though i don't see port being providing any power to any device.

After disabling POE

port goes down.

So, with port status being down no point in running TDR tests, right?

[Leo Laohoo]

---

@royce.varughese.1984 wrote:
with port status being down no point in running TDR tests, right?

---

Always do the TDR with PoE turned off.  

[6502]

So if the developers can’t/won’t fix the bug in IOS-XE, couldn’t they at least add a check for POE being enabled on the port, or at the very least output a reminder to the CLI?

[Leo Laohoo]

---

@6502 wrote:
So if the developers can’t/won’t fix the bug in IOS-XE, couldn’t they at least add a check for POE being enabled on the port, or at the very least output a reminder to the CLI?

---

Long-story-short:  Nobody is going to prioritize or fix CSCvw97924 & CSCwd97177 without any "executive support".  

Read between the lines:  The only time these two bugs are going to get fixed is when there is a very big "whale" who will lean on their Account Team (hard).  Without it, these two bugs are just going to be part of Cisco's "statistics".  

I showed (and shared) several screenshots of TDR going "nuts" to an in-country Cisco SE (not assigned to our account) in 2022 during Cisco Live Melbourne.  He invited a US-based "technical expert" and, together, they looked at the internal comments in the Bug ID, mumbled a few words, closed their laptops and walked away.  That was it.