2.2 Power over Ethernet (PoE, PoE+, PoE++) Standards and Budgeting

Power over Ethernet (PoE) revolutionizes campus network infrastructure by delivering low-voltage direct current (DC) electrical power simultaneously with data over standard twisted-pair Ethernet cabling. By eliminating the necessity for dedicated AC electrical outlets and external power bricks at every endpoint location, PoE dramatically lowers deployment expenses, simplifies installation logistics, and enables centralized power backup via uninterruptible power supplies (UPS) located in telecommunications rooms.

In a PoE deployment, two functional device roles exist:

1. Power Sourcing Equipment (PSE): The device that injects power onto the network cable, typically a PoE-capable access layer switch or midspan power injector.
2. Powered Device (PD): The client endpoint that draws power from the Ethernet cable, such as an IP telephone, wireless access point, pan-tilt-zoom (PTZ) IP surveillance camera, smart lighting fixture, or biometric door lock.

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Evolution of IEEE Power over Ethernet Standards

The Institute of Electrical and Electronics Engineers (IEEE) has formalized four generations of PoE standards to accommodate ever-increasing power requirements of enterprise edge devices:

IEEE 802.3af (PoE / Type 1)

Ratified in 2003, IEEE 802.3af introduced the initial standardized method to deliver electrical power over twisted-pair copper. It supplies up to 15.4 Watts of DC power at the switch port (PSE), delivering a guaranteed minimum of 12.95 Watts to the powered device at the end of a 100-meter channel (accounting for line resistance and heat dissipation). Type 1 utilizes two pairs of a four-pair Category 5 cable and supports hardware Classes 0 through 3. Typical endpoints include basic single-line VoIP phones, stationary IP cameras, and early single-radio 802.11b/g APs.

IEEE 802.3at (PoE+ / Type 2)

Ratified in 2009, IEEE 802.3at nearly doubled power output to support dual-radio 802.11n/802.11ac Wi-Fi access points, video IP phones, and motorized cameras. PoE+ delivers up to 30.0 Watts at the PSE port, guaranteeing 25.5 Watts at the powered device over a 100-meter run. It operates over two pairs of Category 5e or higher cable, introduces hardware Class 4, and is completely backward-compatible with 802.3af devices.

IEEE 802.3bt (PoE++ / 4-Pair PoE / 4PPoE)

Ratified in 2018, IEEE 802.3bt introduced the ability to deliver electrical power across all four twisted pairs simultaneously. Energizing all four pairs cuts cable heating and power transmission losses in half compared to delivering equivalent power over two pairs. The standard defines two distinct operational types:

• Type 3 (High-Power PoE): Delivers up to 60 Watts at the PSE port, guaranteeing 51 Watts at the PD over Cat5e or higher cabling. It introduces Classes 5 and 6 and powers high-end tri-radio access points, video conferencing touchscreens, and building automation controllers. For example, HPE QuickSpecs list the Wi-Fi 7 AP-735 as unrestricted on 802.3bt Class 5 power and restricted (USB disabled) on 802.3at Class 4.
• Type 4 (Ultra-High-Power PoE): Delivers up to 90 Watts (with theoretical maximums approaching 100W) at the PSE port, ensuring 71.3 Watts at the PD over Cat6A cabling. It introduces Classes 7 and 8 and powers motorized PTZ cameras with integrated heaters and blowers, thin client workstations, digital signage displays, and smart LED lighting grids. Campus access points normally fit within Type 3 (Class 5 or 6) budgets.

Comprehensive IEEE PoE Standards Comparison

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PoE Hardware Detection and Classification Mechanics

To prevent damaging non-PoE network interface cards (such as standard laptop LAN ports), Power Sourcing Equipment never applies high voltage directly upon physical cable insertion. Instead, a strict, standardized four-stage handshake occurs:

Stage 1: Detection (Signature Resistance Probe)

The PSE emits a low probing voltage pulse between 2.8 Volts and 10 Volts DC onto the cable and measures the returned electrical impedance. A valid PoE-compliant Powered Device presents an active internal signature resistance of exactly 25 kΩ (23.75 kΩ to 26.25 kΩ). If the switch detects a short circuit, an open circuit, or standard non-PoE termination (typically 75 Ω), the PSE halts power application and operates the port purely as a standard data interface.

Stage 2: Hardware Classification

Once a valid 25 kΩ signature is verified, the PSE ramps probing voltage to between 14.5 Volts and 20.5 Volts DC to determine the device's electrical class. The PD responds by drawing a specific constant current, signaling its maximum required wattage:

Under 802.3bt, the PSE transmits multiple sequential classification pulses ("multi-event classification") to signal to the PD that the PSE is 4-pair capable and to confirm whether 60W or 90W power levels are authorized.

Stage 3: Power-Up

Upon verifying class capabilities and confirming sufficient available power in the switch power budget, the PSE smoothly ramps voltage up to the full operating level (48V to 54V DC) within a few milliseconds.

Stage 4: Dynamic Software Negotiation via LLDP / LLDP-MED

Hardware classification is coarse-grained. For example, a device requesting Class 4 reserves 30W from the switch budget, even if it actually consumes only 8W under typical operation. Once the PD boots its network stack, it exchanges Link Layer Discovery Protocol (LLDP) and LLDP-MED Data Link Layer frames with the switch. Through the Power-via-MDI TLV, the PD communicates its precise power requirement down to 0.1 Watt increments, allowing the AOS-CX switch to dynamically adjust its reserved power budget.

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AOS-CX Switch Power Budgeting and Allocation Policies

Access layer switches operate with finite internal power supplies. In enterprise environments populated with dozens of high-draw access points and video surveillance terminals, unmanaged power consumption can exceed available switch capacity.

Power Supply Configurations and Redundancy

AOS-CX switches feature flexible power supply architectures:

• Fixed Internal PSUs: Found in entry-level access switches (such as the CX 6100 and CX 6200F), providing fixed PoE budgets (e.g., 370W or 740W).
• Modular Hot-Swappable Dual PSUs: Found in modular enterprise switches (such as the CX 6300M and CX 6400). Installing dual power supply units allows administrators to configure:
  • Power Redundancy (N+1): If one PSU fails or an AC feed drops, the surviving PSU maintains power delivery without dropping connected devices.
  • Power Pooling / Aggregate Mode: Combines the output of the installed PSUs into one power pool, maximizing total available PoE wattage for high-density 802.3bt deployments. The exact budget depends on the model and PSU combination listed in the HPE PoE Planning Guide.

Power Allocation Methods: power-over-ethernet allocate-by

AOS-CX configures the allocation method per interface with power-over-ethernet allocate-by {usage | class} (AOS-CX 10.14 CLI Guide, 6300/6400):

1. allocate-by usage (the AOS-CX default):

   • Without LLDP negotiation, the allocated value tracks the port's actual instantaneous draw, and power is reserved based on actual consumption.
   • When the PD negotiates power through LLDP, the LLDP-negotiated value becomes the allocated power.
   • Advantage: Maximizes port density and avoids reserving watts that devices never use.
   • Disadvantage: Requires budget monitoring, because several devices can increase their draw at the same time.

2. allocate-by class:

   • Without LLDP negotiation, the switch reserves power based on the PD's requested class (for example, 30 W for a Class 4 device). With LLDP negotiation, the negotiated value is used.
   • Advantage: Predictable reservations for devices that do not negotiate.
   • Disadvantage: A switch full of Class 4 devices that actually draw 6 W can report an exhausted budget.

3. Capping a port: power-over-ethernet assigned-class {3 | 4 | 6} limits the maximum power a port will deliver, which is useful for non-compliant or legacy devices. (ArubaOS-Switch used poe-allocate-by value; AOS-CX has no "value" method.)

Port Prioritization and Power Shedding

When a switch power supply suffers an AC feed drop, or total instantaneous power demand exceeds available budget, the switch initiates power shedding to prevent a catastrophic chassis brownout. Power shedding disconnects power from ports based on the level set with power-over-ethernet priority {critical | high | low}:

• critical: Highest priority tier. Reserved for mission-critical infrastructure such as emergency VoIP phones, medical alert terminals, and core wireless APs. Ports assigned critical priority are the absolute last to lose power.
• high: Medium priority tier. Assigned to standard enterprise wireless APs and physical access controllers.
• low (Default): Lowest priority tier. Assigned to non-critical desktop peripherals, digital signage, and test devices. Ports assigned low priority are shut down first.

Tie-Breaking Rule: Within the same priority level, lower-numbered ports have higher precedence, so higher-numbered ports lose power first (for example, port 1/1/48 before port 1/1/1).

Always-On PoE and Quick PoE

Traditional switches disrupt PoE output during operating system reboots, forcing all connected APs, security cameras, and VoIP phones to cycle their power and spend several minutes rebooting. AOS-CX provides two power-continuity features (AOS-CX 10.14 CLI Guide):

• Always-on PoE (power-over-ethernet always-on <MODULE-ID>, global context): keeps delivering power across a soft reboot to ports that were already delivering power. It is enabled by default at the switch or slot level. It cannot help if the switch itself loses power.
• Quick PoE (power-over-ethernet quick-poe <MODULE-ID>): after a cold boot, applies power to connected PDs as soon as possible instead of waiting for the full operating system boot. It is disabled by default, and all ports on the subsystem must have PoE enabled before you turn it on.

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AOS-CX CLI Configuration and Diagnostics

Configuring PoE Allocation and Priority

To configure usage-based allocation globally and assign port-specific priorities, enter the following commands:

switch# configure terminal
switch(config)# power-over-ethernet always-on 1/1
switch(config)# power-over-ethernet quick-poe 1/1

switch(config)# interface 1/1/1
switch(config-if)# description "Campus Security Emergency Phone"
switch(config-if)# power-over-ethernet priority critical

switch(config)# interface 1/1/2
switch(config-if)# description "Wi-Fi 6E Access Point"
switch(config-if)# power-over-ethernet allocate-by usage
switch(config-if)# power-over-ethernet priority high

Verifying Global PoE Status

To view overall power capacity, allocated wattage, and real-time power draw, execute show power-over-ethernet. The output below is abbreviated for study purposes; field names vary by release:

switch# show power-over-ethernet

  Total Available Power    :   740.0 W
  Total Allocated Power    :   312.4 W
  Total Remaining Power    :   427.6 W
  Total Real-Time Power    :   186.2 W
  Power Allocation Method  :   usage
  Internal Power Supply    :   OK
  Redundant Power Supply   :   OK

Inspecting Per-Port Power Parameters

To inspect detailed electrical parameters, hardware class, and LLDP-negotiated power on a specific port, execute show power-over-ethernet <PORT>:

switch# show power-over-ethernet 1/1/2

  Port                  : 1/1/2
  Status                : Delivering
  Power Priority        : High
  Admin Status          : Enabled
  Assigned Class        : Class 6 (Type 3 802.3bt)
  Allocated Power       : 45.0 W
  Instantaneous Power   : 22.8 W
  Average Power         : 21.4 W
  Peak Power            : 34.6 W
  Voltage               : 54.1 V
  Current               : 421 mA
  Always-On (module)    : Enabled