How to Troubleshoot PoE : Common Issues and Testing Steps
Selene Gong
Power over Ethernet (PoE) technology plays a vital role in modern network infrastructure by simplifying device deployment — delivering both power and data over a single Ethernet cable. Cisco Catalyst switches, including the widely deployed 9300 and 2960 series, support multiple PoE standards enabling devices like IP phones, wireless access points, and security cameras to operate without dedicated power sources.
Despite its convenience, PoE can sometimes fail or behave unpredictably, causing devices to lose power, intermittently disconnect, or fail to start. This article provides a detailed, step-by-step troubleshooting guide focusing on Cisco Catalyst 9300 switches, supplemented by general principles applicable to other models like the 2960.
Cisco switches function as Power Sourcing Equipment (PSE), supplying power to Powered Devices (PDs) according to several PoE standards:
IEEE 802.3af (PoE): Up to 15.4W per port.
IEEE 802.3at (PoE+): Up to 30W per port; supported by Cisco Catalyst 9000 "P" models (e.g., C9300-48P).
Cisco UPoE: Proprietary extension delivering up to 60W per port using LLDP negotiation; available on "U" model switches (e.g., C9300-24U).
Cisco UPoE+ (IEEE 802.3bt based): Delivers up to 90W per port on select Catalyst 9000 "H" models (e.g., C9300-48H).
PoE devices communicate their power needs through classes (0–8), allowing the switch to allocate power efficiently without exceeding its power budget.
Part 2: PoE Troubleshooting by Error Category
When PoE problems occur, categorizing symptoms into distinct error types helps streamline diagnosis and resolution. Below are 4 common PoE error categories with detailed symptoms, causes, troubleshooting steps, and solutions.
PoE Error 1: No Power to Device
Symptoms:
PD (e.g., IP phone, AP) fails to power on entirely.
show power inline may show device still connected even after unplugging.
Non-PoE devices on the same switch function normally.
Common Causes:
Port administratively shut down or error-disabled.
power inline never command configured on port.
Power budget exhausted or misconfigured.
Faulty or incompatible cabling.
Troubleshooting Steps:
Test the PD on different ports to isolate port-related faults.
Run show interface status and show running-config interface to verify port state.
Ensure power inline never is not set on the interface (show running-config interface ).
Inspect and replace cables; verify cable length ≤ 100m.
Use show power inline and show power inline detail to check available and consumed power.
Confirm that automatic power management is enabled if applicable.
Observe PoE LEDs for amber or off status indicating fault.
Solutions:
Replace cables if defective or non-compliant.
Increase power supply redundancy or add power supplies to meet demand.
Use power inline static max for third-party devices lacking proper classification.
For Class 4 devices without CDP/LLDP support, enable 2-event classification with power inline port 2-event.
PoE Error 2: PD Powers Off or Reloads Intermittently
Symptoms:
Devices lose power intermittently or reboot unexpectedly.
Sudden disconnections correlated with network instability.
Common Causes:
Insufficient power during peak load (e.g., PTZ cameras).
Poor cable quality or excessive cable length.
Intermittent electrical connections.
Network instability triggering device resets.
Troubleshooting Steps:
Measure peak power draw of PDs during operation.
Check cable integrity and length; replace if necessary.
Examine syslog via show logging for error correlations.
Use SPAN or traffic capture to identify network issues impacting PDs.
Verify physical connections and cable termination quality.
Solutions:
Ensure switch power budget covers peak PD consumption.
Use high-quality, correctly rated cables within standard lengths.
Disable LLDP selectively (no lldp transmit) if PDs misbehave with CDP/LLDP negotiation.
Consider enabling Perpetual or Fast PoE features to improve resilience.
PoE Error 3: Partial Powering of PDs on Same Switch or PSE
Symptoms:
Some PDs receive power; others on same switch or power source do not.
Non-PoE devices on same ports function normally.
Common Causes:
Individual ports administratively down or error-disabled.
Power budget depleted, unable to power new PDs.
Faulty ports or hardware issues.
Troubleshooting Steps:
Move non-powered PDs to known working PoE ports.
Check port status with show interface status and verify no shutdown or err-disabled.
Confirm available PoE budget with show power inline.
Review configuration for any per-port power limits.
Solutions:
Enable or re-enable ports using no shutdown.
Redistribute PDs to additional PSE devices if available.
Monitor power allocation and restrict per-port max power if necessary.
PoE Error 4: PoE Cameras Not Powered
Symptoms:
IP cameras fail to power on via PoE switch or injector.
Port LEDs off or inconsistent.
Common Causes:
Compatibility issues between camera and PoE source.
Damaged cables or ports.
Camera PoE module failure.
Troubleshooting Steps:
Verify camera compatibility with switch/injector specifications.
Inspect and test cables and switch ports; swap to known good ports.
Use DC power adapter as a fallback to confirm camera’s power module functionality.
Solutions:
Replace incompatible PoE devices with compatible models.
Ensure cabling and ports are physically intact and clean.
Use direct DC power if PoE is not an option temporarily.
Part 3: Common CLI Commands for PoE Troubleshooting
show power inline — Overview of PoE power consumption and budget.
show power inline detail — Detailed info per port: power class, PD type, admin status.
show interface status — Check port status (up/down, err-disabled).
show running-config interface — Check PoE-related interface config.
show logging — View syslog for PoE-related errors.
show environment all — Verify hardware environment including power supplies.
diagnostic start switch test DiagPoETest — Run PoE diagnostics on Catalyst 9300/9200 (use cautiously).
debug ilpower event and related debugs — Advanced PoE troubleshooting.
Part 4: Model-Specific Notes and Recommendations
Cisco Catalyst 9300
Known issues include PoE devices not powering on dynamically post-boot and inaccurate port status reporting (show power inline showing unplugged devices).
Bugs such as CSCwe22958, CSCwe54104, and CSCwe60763 relate to PoE issues on certain IOS XE versions (e.g., 17.03.03).
Recommended to keep IOS XE updated (e.g., 17.6.5 MD or later).
If problems persist, contact Cisco TAC and consider RMA or hardware inspection.
For stack setups, hard power cycling the affected member may help.
Cisco Catalyst 2960
Less documented for specific PoE bugs in provided sources.
General troubleshooting principles apply: verify power budget, port status, cabling, and PD compatibility.
Consult official Cisco 2960 documentation or TAC for detailed guidance.
Part 5: Real-World Case Studies
Case 1: A newly deployed C9300-48P switch failed to power on devices plugged in after boot. Powering devices connected during boot worked, but show power inline incorrectly showed devices still connected after removal. Other identical switches worked fine on same IOS XE version. Resolution involved TAC case and hardware RMA, suggesting unit-specific faults or bugs.
Case 2: A user experienced %ILPOWER-3-CONTROLLER_PORT_ERR errors with intermittent PD power loss. Devices failed to send "Power Good" signals, causing PoE cycling. Attempts to fix included cable replacement, IOS XE upgrade, and port changes. Advanced debug and TAC support were necessary for resolution.
Part 6: Preventive Maintenance and Best Practices
Power Budget Planning: Calculate total PD power needs with margin for expansion and peak loads.
Quality Cabling: Use certified Cat5e/Cat6 or better, keep runs ≤ 100m.
Environmental Controls: Maintain proper grounding, temperature, and humidity.
Compatibility Checks: Validate non-Cisco PD compatibility and enable LLDP/CDP as needed.
Enable Perpetual/Fast PoE: Where supported, to minimize downtime during network events.
FAQ
Q1: How to troubleshoot PoE?
Start by checking if PoE is enabled on the switch port using CLI (show power inline). Verify the device’s PoE class and power requirements. Inspect cables for damage, ensure they are Cat5e or higher, and test on a known-working port.
Q2: How to test if PoE is working?
Connect a known PoE-powered device (e.g., IP phone, AP) to the port. If it powers on, PoE is working. Alternatively, use a PoE tester or multimeter to measure voltage (typically ~48V DC).
Q3: How to know if a PoE injector is bad?
Test the injector with a known-working device and cable. If the device does not power on but works with another injector, the original is likely faulty. Also check if the injector’s LED indicators are off or showing error status.
Q4: What to be checked if PoE is not getting power supply?
Confirm PoE is enabled in the switch configuration, ensure the total PoE budget isn’t exceeded, check the power supply modules, and replace suspect cables. Inspect logs (show logging) for any PoE-related errors.
By following these structured troubleshooting steps, leveraging Cisco CLI tools, and maintaining good operational practices, network engineers can resolve most PoE issues on Cisco switches efficiently, ensuring reliable power delivery and network uptime.
