Finding a dodgy cable with Cisco Catalyst cable diagnostics

We had a couple of servers that were themselves reporting as connected @ 1G/FD and also the same for the switchports they were connectd to. Yet iperf between the two boxes was pretty pants – about 34MBits/sec.
So I had a think and looked up how to test the cable lengths, which also showed me that the twisted pairs that make up an ethernet cable weren’t all working properly.
After the cable change, iperf between the same two machines showed  941 Mbits/sec, which is a vast improvement!

before cable change

core02#show cable-diagnostics tdr interface gigabitEthernet 1/0/11
TDR test last run on: August 12 19:50:34
Interface Speed Local pair Pair length Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Gi1/0/11 1000M Pair A 7 +/- 10 meters Pair B Normal
Pair B 0 +/- 10 meters Pair A Open
Pair C 1 +/- 10 meters Pair D Open
Pair D 0 +/- 10 meters Pair C Open
core02#show cable-diagnostics tdr interface gigabitEthernet 1/0/12
TDR test last run on: August 12 19:50:50
Interface Speed Local pair Pair length Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Gi1/0/12 1000M Pair A 1 +/- 10 meters Pair A Open
Pair B 7 +/- 10 meters Pair B Normal
Pair C 0 +/- 10 meters Pair C Open
Pair D 0 +/- 10 meters Pair D Short/Crosstalk

after cable change

core02#show cable-diagnostics tdr interface gigabitEthernet 1/0/11
TDR test last run on: August 12 20:10:32
Interface Speed Local pair Pair length Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Gi1/0/11 1000M Pair A 345 +/- 10 meters Pair B Normal
Pair B 345 +/- 10 meters Pair A Normal
Pair C 345 +/- 10 meters Pair D Normal
Pair D 345 +/- 10 meters Pair C Normal
core02#test cable-diagnostics tdr interface gigabitEthernet 1/0/12
TDR test started on interface Gi1/0/12
A TDR test can take a few seconds to run on an interface
Use 'show cable-diagnostics tdr' to read the TDR results.
core02#show cable-diagnostics tdr interface gigabitEthernet 1/0/12
TDR test last run on: August 12 20:11:20
Interface Speed Local pair Pair length Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Gi1/0/12 1000M Pair A 347 +/- 10 meters Pair B Normal
Pair B 347 +/- 10 meters Pair A Normal
Pair C 347 +/- 10 meters Pair D Normal
Pair D 347 +/- 10 meters Pair C Normal

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