A PoE switch not supplying power can result from misconfigured ports, exhausted power budget, faulty cabling, or hardware issues, and can be resolved through systematic checks and adjustments.

Step 1: Verify Port and PoE Configuration

Ensure the affected port is administratively up and not error-disabled. Use commands like show interface status or show power inline to confirm the port is active and PoE is enabled. On Cisco switches, the command power inline auto enables PoE on a port, while power inline never disables it, which can prevent devices from receiving power . For Juniper switches, flapping the interface with set interface x/x/x disable followed by enable can reset PoE functionality .

Step 2: Check Power Budget

PoE switches have a total power budget. If the connected devices exceed this limit, new devices may not receive power. Use show power inline to review the total and remaining budget. If the budget is exhausted, consider redistributing devices across ports or upgrading the power supply .

Step 3: Inspect Cabling and Device Compatibility

Verify that Ethernet cables are PoE-compliant (Cat5e or Cat6 recommended) and within the 100-meter limit. A mismatch between the PoE mode of the switch (Alternative A, B, or 4-pair) and the powered device can prevent power delivery. Test the powered device on a known working port with a short patch cable to isolate cable or device issues .

Step 4: Review Logs and Firmware

Check switch logs using show log or SNMP traps for alarms or errors. Firmware bugs can affect PoE operation, so ensure the switch is running a stable, updated IOS or firmware . Persistent alarms or errors like Imax or Tstart may indicate negotiation failures or internal PoE controller issues.

Step 5: Hardware Diagnostics

If all ports fail to provide power but non-PoE devices work, the PoE power supply or controller may be defective. For field-replaceable units, consider replacing the power supply. If only a subset of ports is affected, the PoE subsection may be faulty . In cases of power surges or outages, the PoE controller can be damaged, requiring hardware replacement or RMA .

Step 6: Test and Isolate

Connect a known working powered device directly to the switch port using a short, compliant cable. If it powers on, the issue may be with the original cabling or device. If it still does not power on, the problem is likely with the switch port or PoE subsystem . By following these steps—verifying configuration, checking power budget, inspecting cabling, reviewing logs, updating firmware, and testing hardware—most PoE power issues can be diagnosed and resolved efficiently.