PoE 802.3bt Power Budget Planning for Access Switches

PoE has quietly become the largest electrical load in many access closets. Wi-Fi 6/7 access points, pan-tilt-zoom cameras, digital signage and building sensors all draw from the switch, and the failure mode is not usually a dead port - it is a switch that cannot honour its commitments when one power supply fails or when every device powers up after an outage. This is how to size it so that does not happen.

The Power Classes You Are Buying From

Note the gap between PSE and PD figures: the difference is cable loss, and it is why a '60 W' port does not deliver 60 W to the device. When a vendor specifies an AP at 45 W, that is PD-side consumption.

Doing the Budget Properly

Step 1 - inventory by class:
  40 x AP @ 45 W (Type 3)       = 1800 W
   8 x PTZ camera @ 25 W        =  200 W
   6 x phone @ 12 W             =   72 W
                                  --------
  Peak PD load                  = 2072 W

Step 2 - check the switch budget in three states:
  Total PoE budget (all PSUs)   = 2880 W   OK
  Budget with one PSU failed    = 1440 W   FAILS at 2072 W
  Per-port limit (class)        = 60 W      OK

Step 3 - add headroom for startup inrush and future density: +15-20%.
  Target >= 2486 W with N+1 -> need a bigger platform or load-shedding design.

That N+1 column is where most designs fail. Access switches are usually deployed with dual power supplies for redundancy, and the redundancy is only real if the PoE budget survives a single PSU loss. Check the datasheet's 'PoE budget with one power supply' figure, not just the headline number.

Load Shedding and Priority

Most enterprise platforms support PoE priority per port, and the disciplined design assigns it: infrastructure and phones first, cameras next, discretionary devices (signage, guest APs) last. When the budget is exceeded, the switch then sheds the right things.

# Cisco IOS example
interface GigabitEthernet1/0/10
 power inline auto max 45000
 power inline port priority high
!
interface GigabitEthernet1/0/20
 power inline auto
 power inline port priority low

The prioritisation only works if it is deliberate. Default priorities are usually equal, which means an arbitrary device gets dropped when the budget trips.

Monitoring, Because the Budget Changes

Poll and alert on four things: total PoE consumption per switch, consumption as a percentage of the N+1 budget, per-port power draw for the top consumers, and PoE fault or denial counters. The denial counter is the one people miss - it increments when a device is refused power, so a camera that 'randomly stops working' is often a port that never got power at all.

show power inline
show power inline module 1
show power inline GigabitEthernet1/0/10 detail
show power inline police        # logs and/or err-disables on overload (platform dependent)

Cabling and Heat

Two physical constraints that bite at 802.3bt rates: 4-pair power delivery needs all four pairs to be intact, so a partially faulty Cat 5e run that worked fine at 15 W may fail to negotiate Type 3; and 90 W per port across 48 ports is north of 4 kW of heat in a closet designed for a tenth of that. Verify cooling and power feeds before the deployment, not after.

Related reading: 华为交换机 PoE 供电与功率管理, multigigabit cabling at 2.5G/5G, and UPS power monitoring over SNMP so that the closet's electrical state is visible alongside the switch's.

原文链接：https://ethernetalliance.org/technology/poe/