Power Over Ethernet Issues

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Cabling Issues

• cabling for PoE+ must be Cat 5e or better
  • standards recommend use of Cat 6A
• drawing power down cable generates more heat
  • if heat is not dissipated, it can affect data rates
  • thermal performance is improved by using pure copper cabling with thicker conductors
    • thin conductor will generate more heat through resistance
    • shielded cabling is capable of dispersing heat more effectively

Incorrect Standard

• Powered device (PD) should be able to negotiate the correct mode and power output with the switch
  • can fail with device that only support the first PoE standard
    • especially if interface is enabled for high power PoE++ type 4 PDs
• switch and PD must negotiate a compatible mode:
  • Alternative A delivers power with data over pairs 1/2 and 3/6
    • compatible with 10/100 and 10/100/100 links
  • Alternative B delivers power over the 10/100 spare pairs (4/5 and 7/8)
    • not compatible with Gigabit Ethernet
  • Four-pair delivers power over all pairs
    • required by PoE++ type 3 and type 4 PDs
    • compatible with 10/100/1000 and supports 10G

Power Budget Exceeded

• each switch has a total power budget for all ports
  • typically around 300-400 watts
  • if exceeded:
    • some devices will not be activated
    • might be intermittent resets
• use show power inline command to report the power budget and power consumption
• may need to provision another switch or use power injector devices for selected PDs
• actual power consumption can fluctuate widely