Frequently Asked Question
My UniFi SFP or DAC uplink stays disconnected. How do I isolate the cause?
QUICK ANSWER
Match the two port capabilities, transceiver or DAC, cable type and link speed. An inserted module is not proof of a working link, and a 10 Gbps label does not mean every port negotiates every lower speed.
Applies to: UniFi SFP/SFP+/SFP28 links; optical and copper modules have different requirements.
Work through these checks
- Record both device and module model numbers and port roles. Confirm that the cable/module supports a speed available on both ends. Check that the port is enabled and assigned the intended WAN or LAN role.
- For RJ45 modules, seat the module fully before connecting the Ethernet cable. For fibre, check the required fibre mode, wavelength pairing, connector cleanliness and transmit/receive orientation without looking into an optical port.
- Compare configured link speed at both ends. Only select a manual speed that both the port and module support. Test with a known-good compatible DAC or matching module/cable pair during an approved interruption.
- Change one component at a time. A short known-good copper patch cable can help distinguish a long-cable fault from a module issue. Record link state and errors after each test.
Confirm the result
The link remains up at the intended speed and carries a sustained local test without recurring disconnects or excessive interface errors.
Before going further: Do not force an unsupported speed or assume another hardware revision is identical. Use appropriate ESD handling and never view a live fibre end.
If you need help
Include these details in your ticket: Both devices/ports, module/DAC part numbers, speed settings, cable/fibre type and swap-test results.
Updated 261107