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Cisco CCNA (200-301)Network FundamentalsMedium

A network administrator is installing a wireless access point that requires up to 25.5 W of power delivered over its Ethernet cable. Which PoE standard provides sufficient power for this device?

  1. A802.3af (PoE)
  2. B802.3u (Fast Ethernet)
  3. C802.3at (PoE+)
  4. DUSB power delivery
Show answer & explanation

Correct answer: C. 802.3at (PoE+)

IEEE 802.3af (PoE) supplies up to 15.4 W at the source, delivering about 12.95 W to the device, which is insufficient. IEEE 802.3at (PoE+) supplies up to 30 W at the source, delivering up to 25.5 W to the powered device, meeting the requirement.

Why the other options are wrong

  • A. 802.3af only guarantees about 12.95 W delivered to the device, too low for this AP.
  • B. 802.3u is the Fast Ethernet standard and has nothing to do with power delivery.
  • D. USB power delivery is not an Ethernet-based PoE standard.

PoE Standards (802.3af/at)

Power over Ethernet standards define how much electrical power can be delivered to devices over Ethernet cabling; 802.3af (PoE) supports up to ~12.95W delivered, and 802.3at (PoE+) supports up to ~25.5W delivered.

  • 802.3af: 15.4W at source, ~12.95W at device
  • 802.3at: 30W at source, ~25.5W at device
  • 802.3bt: up to 71-90W for high-power devices
  • Common for APs, IP phones, cameras

Memory trick: 'af' = a few watts, 'at' = a ton more watts.

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