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WS-C3850-24XU PHY Loopback: loopback Test : End, Status Failed


Hi All,

We have a WS-C3850-24XU failed on PHY Loopback: loopback Test. 
The switch is stand anlone and is not connect to network.

I run the command : diagnostic start switch 1 test all.

After the switch finish the test and reboot I run the command : Show dianostic post there are 3 port failed.

Switch 1

POST: MBIST Tests : Begin
POST: MBIST Tests : End, Status Passed

POST: CRYPTO Tests : Begin
POST: CRYPTO Tests : End, Status Passed

POST: PHY Loopback: loopback Test : Begin
POST: PHY Loopback: Failed For Interface : TenGigabitEthernet1/0/1
POST: PHY Loopback: Failed For Interface : TenGigabitEthernet1/0/7
POST: PHY Loopback: Failed For Interface : TenGigabitEthernet1/0/15
POST: PHY Loopback: loopback Test : End, Status Failed

There are 3 amber lights on TenG1/0/1 ,TenG1/0/7 and TenG1/0/15.

The switch image version: 16.6.4
I downgrade the switch image version to :16.6.1 but the loopback Test still failed on TenG 1/0/1

I downgrade the switch image version to : 16.3.7 but there are 2 port failed on TenG 1/0/3 and TenG 1/0/24 

Is this a software issue or hardware issue ?
I have run the diagnostic test on many WS-C3850 switches and I have not encountered the  problem like this WS-C3850-24XU.
Please advise.

4 Replies 4

Leo Laohoo
Hall of Fame
Hall of Fame
Contact Cisco TAC and organize for an RMA.


Check the below link, it has a good description of your issue. 



Sorry for the late reply,Leo.
I configured port TenG1/0/1,1/0/7 and 1/0/15 to routed ports on the WS-C3850-24XU and then configured port Gig1/0/1,1/0/7 and 1/0/15 to routed port on a WS-C3850-48P-E.
The WS-C3850-24XU 3 ports LED light were amber and both switches can ping.
I also read the link B.kablawi90 has provided ( Thanks,B.kablawi90) the WS-C3850-24XU loopback test failed seem to be relate to software issue. I had used the diagnostic test for 3 different IOS Images on the same switch and the failed test port seem to be bouncing around.
If the failed test port is hardware related, no matter what IOS Image I used the failed test port should be the same port.
I could be wrong.
What is your suggestion, Leo ?

Do something radical ... If this is a "software issue", upgrade the firmware to something really high.
If the reboot still comes back with the same "loopback failure", then contact Cisco TAC.
Personally, I would just contact Cisco TAC and organize an RMA.
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