PoE Power Budget Mismatch¶
The Problem¶
My USW-Flex-2.5G-5 switch kept going offline and coming back, cycling every few minutes. The UniFi controller showed it as disconnected, then connected, then disconnected again. This went on for weeks.
The Red Herrings¶
I chased several wrong theories:
- Cable quality — Swapped the cable between the Flex and the 8-port switch. No change.
- Firmware bug — Checked for updates, rebooted the Flex. No change.
- VLAN misconfiguration — Spent hours reviewing port profiles and trunk settings. Everything looked correct.
- Spanning tree — Suspected an STP loop was causing the flapping. Checked topology — no loops.
The real clue was in the timing. The Flex would work for a few minutes, draw more power as traffic increased, hit the limit, and the upstream switch would cut power. After a brief cooldown, it would re-power and start the cycle again.
The Root Cause¶
PoE standards mismatch.
| Standard | Max Power per Port | What Uses It |
|---|---|---|
| 802.3af (PoE) | 15.4W | IP phones, basic APs |
| 802.3at (PoE+) | 30W | NanoHD APs (~10.5W each) |
| 802.3bt (PoE++) | 60-90W | USW-Flex-2.5G-5 (~20-25W) |
The USW-Lite-8-PoE only supports 802.3at (PoE+). While it can deliver up to ~30W per port, the Flex 2.5G draws 20-25W at peak, which should theoretically fit — but in practice, the PoE negotiation and overhead meant the Lite-8 couldn't reliably supply what the Flex needed.
The Fix¶
Two options:
- Power the Flex via USB-C — The Flex has a USB-C power port. A standard USB-C PD adapter bypasses PoE entirely. This is what I did.
- Use a PoE++ source — An injector or switch that supports 802.3bt.
The Lesson¶
When a PoE device intermittently drops, always check the power standard it requires versus what the source provides. The UniFi controller doesn't always warn you about this mismatch — it just shows the device as offline.
Also: the total PoE budget matters. The USW-Lite-16-PoE has a 45W total budget across all ports. With two NanoHDs (~21W combined) and cameras, you can exceed that budget quickly. Plan your PoE power budget the same way you'd plan compute resources.