Common PoE Problems and Troubleshooting

• Insufficient power budget - Switch cannot provide enough watts to all connected PoE devices

  • Each PoE port reserves power even if device doesn’t need full allocation
  • For example, IP phone may only use 7W but switch reserves 15.4W (PoE standard)
  • Solution: Calculate actual power consumption vs. switch capacity

• Cable distance limitations - PoE power degrades over distance due to resistance

  • Standard Ethernet 100m limit still applies, but power may be insufficient before reaching distance limit
  • Voltage drop increases with cable length and gauge (thinner cables = more resistance)
  • Use Cat6/Cat6a for better power delivery over longer runs

• Incompatible PoE standards - Device and switch must support compatible PoE types

  • Device requesting PoE++ (60W) won’t work properly on PoE+ (25.5W) switch
  • Always verify power requirements match switch capabilities

• Faulty cable pairs - PoE requires specific wire pairs for power delivery

  • Alternative A: Uses data pairs 1,2 and 3,6 for power
  • Alternative B: Uses spare pairs 4,5 and 7,8 for power
  • Single damaged pair can cause power delivery failure even if data works

PoE Standards and Power Delivery

PSE = Power Sourcing Equipment (switch), PD = Powered Device

Troubleshooting Commands and Methods

• Verify PoE status: show power inline - displays per-port power consumption and availability
• Check power budget: Look for “Available” vs “Used” power in switch output
• Test cable integrity: Use cable tester to verify all 8 wires have continuity
• Monitor power negotiation: show power inline <interface> for detailed port status

Power Negotiation Process

• Device detection phase - Switch sends low voltage to detect PoE device (25kΩ resistor signature)
• Classification phase - Device indicates power requirements through current draw
• Power delivery phase - Switch provides 48V DC if sufficient power available
• If negotiation fails, device may not power on or operate intermittently

Vocabulary

• PSE (Power Sourcing Equipment): Device that provides PoE power (typically switches or injectors)
• PD (Powered Device): Device that receives PoE power (IP phones, cameras, APs)
• Power budget: Total watts available on switch for PoE devices
• Midspan/Endspan: Midspan = separate PoE injector, Endspan = PoE-capable switch
• LLDP-MED: Protocol extension that communicates actual power requirements (more efficient than standard classification)

Notes

• Always calculate total power requirements before deploying PoE devices - switches will shut down ports if power budget exceeded
• PoE works on straight-through and crossover cables, but not on rollover cables
• Some devices support power stacking where unused PoE+ ports can combine power for higher-wattage devices
• Legacy devices may not follow IEEE standards - verify compatibility before deployment
• Consider environmental factors - high temperatures reduce power delivery efficiency
• Use PoE injectors for non-PoE switches, but verify they don’t interfere with existing network infrastructure
• Never connect PoE output to non-PoE input - can damage equipment even though detection should prevent this