How to fix error message “loopback error detected on 3850 Switches when you plug in an ethernet cable, this most likely happens when it is a circuit that is provided by the internet carrier and the cable is going to the modem
when the ethernet cable is plugged in – the switch starts negotiating and sends keepalive messages – the message is looped backed to the switch and the switch puts the port in the err-disable mode
The workaround to disable keepalive is as follows
- Log in to the switch
- “config t” to go the config mode
- use the interface command to go to the interface config mode “interface gig 0/1” and type “no keepalive”
This should do the trick and always check your interface settings to see how it is negotiated
