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Something very strange here. Today I noticed a device was unreachable. Giving a look at the switch it connects to (Cisco 3750, version 12.2(35)SE5), the port was in err-disabled due to link-flap:

#sh int g1/0/15 status err-disabled

Port      Name               Status       Reason
Gi1/0/15                     err-disabled link-flap

Trying a shut no shut on the interface, puts it again after a while on err-disabled due to link-flap. Thinking of a possible physical problem I've run the (usually overlooked) tdr which surprisingly reported the cable as Normal despite estimated as 65535 meters long and no pair on the other end.

#sh cable-diagnostics tdr int g1/0/15
TDR test last run on: March 01 01:11:00

Interface Speed Local pair Pair length        Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Gi1/0/15  100M  Pair A     65535 +/- 4  meters N/A         Normal
                Pair B     65535 +/- 4  meters N/A         Normal
                Pair C     65535 +/- 4  meters N/A         Normal
                Pair D     65535 +/- 4  meters N/A         Normal

tdr usually returns me useful values, but there's clearly something very wrong here. The number suggests me a kind or unhandled condition due to software bug.

In case of bad cable, tdr should just return Open or Short for affected pair(s). All other ports show correct results for tdr, in all cases (cable unplugged, cable plugged on switch only, cable ok, faulty cable).

Why is this test failing? Has anyone ever experienced this?

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  • Which IOS version are you using? – Ron Maupin Apr 2 '20 at 19:48
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    65535 is "-1". The test is failing, or the cable is unplugged. – Ricky Apr 2 '20 at 21:32
  • @RickyBeam the cable is plugged as the port goes err-disabled. And the reason why the test is failing is the purpose of my question. – kuma Apr 3 '20 at 16:47
  • What does the log say is the reason for the err-disabled? – Ron Trunk Apr 3 '20 at 16:53
  • @RonTrunk link-flap. In fact, when I restart the port it quickly goes up and down several times until it's put in err-disabled – kuma Apr 3 '20 at 16:55

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