pjhtech Tools
UniFi

UniFi Ethernet Uplink Stuck at 100 Mbps

Diagnose FE instead of GbE on an AP, switch or gateway: verify negotiated speed, isolate cable and port faults, and separate link rate from Wi-Fi throughput.

If a gigabit-capable AP uplink shows FE/100 Mbps, set both ends to Link Speed → Auto and replace a faulty cable or 100 Mbps injector. First check the actual port/injector model: an Ethernet setting cannot upgrade 100 Mbps-only hardware.

Read the negotiated speed at each hop

In UniFi Devices → switch → Ports → Port Manager, select the AP port and read its negotiated link rate (FE/100 Mbps versus GbE/1 Gbps). Repeat at each uplink toward the gateway and at the peer port.

Compare each port/injector/adapter’s maximum Ethernet speed in its exact model specification; a gigabit AP attached through a 100 Mbps injector remains limited to 100 Mbps.

Check automatic negotiation

For the affected port, open Advanced → Link Speed in Port Manager. If one end was accidentally forced to 100 Mbps, set Link Speed → Auto at both ends where supported and apply. Link changes interrupt the affected uplink.

Read the negotiated rate again. If it remains at 100 Mbps despite gigabit capability at both ends, continue with the cable and port tests below. Forcing 1 Gbps does not repair a faulty cable.

Auto still negotiates 100 Mbps: find the limiting component

Compare a short known-good cable, then a compatible known-good port. Include patch leads and wall terminations. Preserve the AP’s management/client VLANs and PoE mode when moving it. The offline-device procedure uses the same cable/port isolation.

  • If a short replacement cable restores gigabit speed, test the original patch leads and wall run separately to locate the faulty segment.
  • If the same device and cable reach gigabit on another compatible port, compare the original port’s speed setting and test it with a second gigabit device.
  • If the original device stays at 100 Mbps on a known-good cable and port, connect a second gigabit device to that same path. If only the original device is slow, inspect its Ethernet connector and model-specific link settings. Test any intermediate injector or adapter separately with a compatible gigabit replacement.

Confirm stability, then measure again

Check negotiated speed, link flaps and errors at both ends during the transfer. Restore the intended port configuration and remove temporary test connections; keep a verified faulty cable out of service. If link rate is correct but throughput remains low, investigate rate limits, load and the rest of the path separately.

References