PoE Standards Explained: 802.3af, 802.3at, 802.3bt PoE++

Access points, cameras, phones and door controllers all want power over the same cable that carries their data, and the three PoE standards behind that convenience carry very different budgets. Mismatch them and you get APs that boot-loop under load, cameras that drop frames in low light, or a switch whose power supply is the quiet bottleneck for the whole floor. This guide walks through what 802.3af, 802.3at and 802.3bt actually deliver, the power classes you negotiate, and how to size a switch power budget before ordering it.

The three standards at a glance

The gap between "PSE per port" and "guaranteed at device" is cable loss, which is why the standards quote two numbers. Anything above ~60 W requires all four pairs - 4PPoE - which in turn means the cabling must be Cat 5e or better with all pairs terminated and tested.

Power classes and negotiation

! Catalyst / IOS XE: inspect what the switch negotiated
show power inline
show power inline GigabitEthernet1/0/5 detail
show power inline police
show environment power          ! chassis power supply budget

! ArubaOS-CX: per-interface PoE detail and class
show power-over-ethernet
show power-over-ethernet brief
show power-over-ethernet interface 1/1/5

There are two mechanisms. The physical layer "class" method (hardware classification) signals the class with a resistor and settles within milliseconds; LLDP power negotiation is a protocol conversation that can report actual consumption and let the PSE allocate accordingly. Modern APs use LLDP to request their real draw, which is why show power inline sometimes reports the class-based value instead of the LLDP value - read the detail view, not just the summary.

Sizing the power budget

budget_required = sum(per_port_allocation x ports_in_use) + margin
# example: 24 APs requesting 25.5 W each
#   25.5 W x 24 = 612 W of PoE demand
#   a 48-port switch with a 740 W PoE budget leaves ~128 W of headroom

Vendor "PoE budget" numbers usually describe the total across the chassis, not per port, and are often quoted for one power supply configuration. If your switches run on one PSU instead of two, or are powered by a smaller PSU, re-check the budget in the datasheet - a 48-port switch advertised at 1440 W may only offer 600 W in the configuration you are ordering. Redundancy matters too: losing one PSU in a 2-PSU switch can drop ports beyond the remaining budget, taking APs offline exactly when the network needs them.

Power priority: what to shed first

Switch(config)# interface GigabitEthernet1/0/5
Switch(config-if)# power inline auto
Switch(config-if)# power inline port priority high
Switch(config-if)# power inline police action log     ! log rather than err-disable
Switch(config-if)# power inline consumption 15400 mW  ! static allocation

Priority decides which ports keep power when demand exceeds the budget. Assign critical to the uplink switches' own management links and life-safety devices, high to core wireless and cameras, and default to desk phones and convenience devices. Also set police action log during rollout: the default err-disable behaviour punishes you for a mis-sized budget by shutting the port, which looks exactly like a broken cable.

Deployment checklist

Terminate all four pairs on every run above 30 W, test the cable after termination, verify the class in the switch detail output for each AP/camera type, and record the actual consumption per port for a week before declaring the budget safe. Then monitor it - a new AP model with a bigger radio can silently push a floor over its budget months after the rollout. Related reading: Aruba AOS-CX initial configuration, voice VLAN for IP phones, transceiver types and compatibility and switch selection criteria.

原文链接：https://en.wikipedia.org/wiki/Power_over_Ethernet