I was trying to connect two switches on different floors
lvl1 sw -> lvl2 sw
When I plugged in the fibre I could see the light was making it to the upstairs but when plugging the fibre cable into the switch on lvl2 the port was going into err-disable with the reason as link flap.
The cause of the issue was the wrong type of fibre cable was used in the fibre patch. Single mode cable was used. When my switch transceiver (10Gig-SR multimode) and fibre patches leads were OM3 multimode.
The core of single mode fibre is much narrower than multimode so they can't work together too much light (signal) is lost.
There are two fixes and both will probably cost you:
1 - re-run the fibre patch cables with multimode cable (OM3 or higher). Range is 300m so this is what is used inside most buildings. If you need a cable run longer then you have to use single mode.
2 - Buy single mode transceivers LRM and single mode patch cables. The single mode transceivers and cables are more expensive.
https://en.wikipedia.org/wiki/Single-mode_optical_fiber
