If a PoE switch shows network connectivity but fails to power devices, the issue is usually related to power budget, port configuration, cable quality, or device compatibility.

Common Causes

1. Insufficient PoE Budget Each PoE switch has a total power budget. If the connected devices collectively require more power than the switch can supply, some devices may receive data but no power. Check the switch's total and per-port power allocation to ensure it meets the PD's requirements .
2. Port Configuration Issues PoE may be disabled on the port or the port may be administratively shut down. On Cisco switches, for example, the command show power inline can indicate if the port is connected but unpowered. Ensure PoE is enabled with commands like power inline auto and that the port is not error-disabled .
3. Cable Problems Poor-quality or damaged Ethernet cables can prevent power delivery while still allowing data transmission. Use Cat5e or higher cables, and for higher PoE standards (PoE+, PoE++), Cat6 or Cat6a is recommended. Cable length should not exceed 100 meters; longer runs may require PoE extenders .
4. Device Compatibility and PoE Standards The powered device (PD) must be compatible with the PoE standard provided by the switch (e.g., IEEE 802.3af, 802.3at, or 802.3bt). Mismatched modes (Alternative A/B or 4-pair) can prevent power delivery even if the device shows a network link .
5. Hardware or Switch Faults Faulty switch ports or internal PoE components can cause power failure. Testing the PD on a different port or using a known working device can help isolate the problem .

Troubleshooting Steps

1. Verify Port Status
   • Check if the port is administratively up (no shutdown) and PoE is enabled (power inline auto).
   • Use show power inline to confirm the port's power status.
2. Check Power Budget
   • Ensure the total power required by all connected devices does not exceed the switch's available PoE budget.
   • Consider reducing the number of devices or upgrading to a higher-capacity switch if necessary.
3. Inspect and Replace Cables
   • Confirm cable type and length. Replace damaged or non-compliant cables.
   • For long runs, consider PoE extenders or higher-quality cables.
4. Test Device Compatibility
   • Verify the PD supports the PoE standard and mode provided by the switch.
   • If possible, test the device on a different PoE switch or port.
5. Reset or Reconfigure the Switch Port
   • Disable and re-enable PoE on the port.
   • Reboot the switch if necessary to clear transient errors. By systematically checking power budget, port configuration, cabling, and device compatibility, most PoE issues where a device shows a network signal but receives no power can be resolved .