99% of the time, the problem is fiber polarity — specifically, Transmit (Tx) talking to Transmit and Receive (Rx) talking to Receive instead of Tx ↔ Rx. Good news: it's incredibly easy to understand and fix once you know the “two-lane highway” rule. I suspect it might be a single-mode SFP, as I wouldn't see the 9-port switch light up otherwise. This seem to go against all my experience with networking but I am only starting the backend learning so I suspect there is something I am missing. Fiber is full-duplex, which means it always uses. I have a strange problem I have not come across before, where one end of a fibre connection shows as "connected" but the other end shows as "not connected". I have a C2960-48PST-L connected to a C3750X-12S-E via OM4 fibre. This article will guide you through the process of troubleshooting fiber optic connections, with a focus on ensuring proper TX and RX alignment and how to correctly switch patch. Fiber optic troubleshooting is an essential skill for network administrators, technicians, and engineers responsible for maintaining and repairing fiber optic systems.