%DTP-5-NONTRUNKPORTON: Port Gi2/0/2 has become non-trunk

[Rdph8214]

Yesterday we had an issue, one of our servers went down and we didn't understand why, looking at the logs today in one of the ports that connects the server with our switch we got the following message

Aug 23 19:24:50.346: %DTP-5-NONTRUNKPORTON: Port Gi2/0/2 has become non-trunk
Aug 23 19:52:00.622: %DTP-5-TRUNKPORTON: Port Gi2/0/2 has become dot1q trunk

searching a bit this error indicates that the port changed from trunk to access.

If no new configuration has been made, why is the switch making this change?

The port configuration is as follows

interface GigabitEthernet2/0/2
switchport mode trunk
switchport nonegotiate
logging event trunk-status
load-interval 30
spanning-tree portfast trunk
ip dhcp snooping trust
end

The switch model is WS-C3850-24P IOS 16.12.07

[marce1000]

                    >...searching a bit this error indicates that the port changed from trunk to access.

  - I don't think so , it is an inclusive message that also appears when the port goes down (if trunk mode is being used) , check port counters, and at server side check networking logs such as eventvwr in Windows (e.g.)

 M.

[balaji.bandi]

what is the other side connected ? what log do you see on another side?

can you post show interface gig2/0/2 (both sides?)

suggest checking Physical Cables also.

[Rdph8214]

Hello,

The other site is a Hyper V server

[balaji.bandi]

HyperV - what you see the Logs ?

how frequent this interface going up and down ?

[Rdph8214]

There is no log to indicate that the port is down, although there is no CRC error requesting to change the cable.

[balaji.bandi]

how far is the server to switch ? any MDF ?

[marce1000]

              - Ref : https://www.vembu.com/blog/hyper-v-event-logs-troubleshooting/

   Pay attention to parts mentioning Hyper-V-VmSwitch (logs) and or examine them.

 M.

[MHM Cisco World]

sorry 
you already share the CRC error 
OK 
check the interface error counter 

if you check and there is no error counter then 
second step is check the Queue, the DTP is traffic with priority 0 and if there is no space in Queue the SW drop the frame and one of drop frame is DTP. 
this explain the port no down but the status is change. 
but waiting your answer to full sure this issue here. 
if you check then you can change the priority 0 queue using the link below 
https://www.cisco.com/c/en/us/support/docs/switches/catalyst-3850-series-switches/200594-Catalyst-3850-Troubleshooting-Output-dr.html

[Jitendra Kumar]

Turned out to be a faulty patch cord causing the problem

Use the new Patch cord.

[Rdph8214 · Accepted solution]

Hello all,

Thanks for your help, we have discovered that the error was human, a technician inadvertently configured that port as monitoring and that generated the error.