Fibre link up/up on side and down/down on other.

[inderjit21]

I have 2x3850 stack connected via fibre . But 1 link has gone not connected sometime back.

I did a shut/no shut on sw01 Te1/1/2 . It showed connected and then went down again.

sw01

Te1/1/2   sw03_gi1/1/1 connected    trunk        full   1000 1000BaseSX SFP
Te2/1/2   sw03_gi2/1/1 connected    trunk        full   1000 1000BaseSX SFP

sw03

Gi1/1/1   sw01_Te1/1/2 notconnect   1            full   1000 1000BaseSX SFP
Gi2/1/1   sw01_Te2/1/2 connected    trunk        full   1000 1000BaseSX SFP

[Philip D'Ath]

Are you sure these connect to what you think they do?

Have you tried popping out the not-connected SFP and putting it back in and seeing what comes up in the log?

If you swap the fibres over on sw03 does the issue follow the SFP or the fibre?

[cihanyilmazer]

Hello,

If your fibre cable is damaged you can faces with this issue. Also, if there is a mismatch on your duplex/speed configuration, firstly link will come up and then will go down. I see that your configuration is not Auto Duplex/Speed. Please set duplex/speed to auto on the interface that has problem and try again. Also, I see that SW3 Gi1/1/1 switchport mode is access but the other end is trunk. If you forced these interfaces as trunk and access, they cannot negotiate to establish a trunk link between them. As a result; please set your interfaces duplex/speed to auto and switchport modes to trunk or access based on your need.

Thank you!