Role of LLDP in PoE Power Negotiation

Introduction

In modern Power over Ethernet (PoE) systems, power delivery is no longer a fixed one-way process.
As devices become more advanced — from Wi-Fi 6 access points to multi-sensor IP cameras — their power requirements change dynamically.

To handle this flexibility, the Link Layer Discovery Protocol (LLDP) plays a vital role.
Defined under IEEE 802.1AB, LLDP enables intelligent, two-way communication between PoE power providers (PSE) and power consumers (PD).

By understanding how LLDP works within the PoE power negotiation process, network designers can ensure optimal performance, energy efficiency, and system safety.

1. What Is LLDP (Link Layer Discovery Protocol)?

LLDP is a Layer 2 (Data Link Layer) protocol that allows Ethernet devices to advertise their identity, capabilities, and configuration to directly connected neighbors.

Each device sends LLDP Data Units (LLDPDUs) at regular intervals, containing key information such as:

• Device name and type
• Port ID and capabilities
• VLAN configuration
• Power requirements (in PoE-enabled devices)

When used with PoE, LLDP is extended through LLDP-MED (Media Endpoint Discovery) or IEEE 802.3at Type 2+ power negotiation extensions, enabling dynamic power communication between PSE and PD.

2. LLDP in the Context of PoE Standards

Before LLDP was introduced, IEEE 802.3af (PoE) used a simple classification system during the initial link-up:

• The PD would indicate its class (0–3)
• The PSE would allocate a fixed power limit (e.g., 15.4 W)

However, as devices evolved, this static approach became insufficient.
For example, a dual-band wireless AP might need 10 W in idle but 25 W under heavy load — impossible to manage efficiently using only the legacy class method.

That’s why IEEE 802.3at (PoE+) and IEEE 802.3bt (PoE++) introduced LLDP-based power negotiation.

3. How LLDP Enables PoE Power Negotiation

The LLDP negotiation process occurs after the physical PoE link is established and the PD has been detected.
Here’s how it works:

Step 1 – Initial Detection and Classification

• The PSE detects a valid PD signature (25kΩ).
• It applies initial power based on the PD class (e.g., Class 4 = 25.5 W).

Step 2 – LLDP Exchange

• Once Ethernet data communication starts, both devices exchange LLDP frames.
• The PD sends its exact power needs (e.g., 18 W for standard mode, 24 W for full operation).
• The PSE replies, confirming available power per port.

Step 3 – Dynamic Adjustment

• The PSE adjusts power output accordingly in real time.
• If multiple PDs compete for power, the PSE prioritizes based on available power budget.

Step 4 – Continuous Monitoring

• The LLDP session continues periodically, allowing the PD to request more or less power as needed.
• This ensures safety, prevents overload, and supports energy efficiency.

4. Advantages of LLDP Power Negotiation

5. LLDP vs Traditional PoE Classification

In short:

Class-based power assignment is static. LLDP-based negotiation is intelligent.

For modern deployments — Wi-Fi 6/6E APs, PTZ cameras, or IoT hubs — LLDP is essential to fully utilize PoE+ and PoE++ capabilities.

6. LLDP in IEEE 802.3bt (PoE++)

Under IEEE 802.3bt, LLDP becomes a core part of the power negotiation process, especially for Type 3 and Type 4 PSE/PD pairs delivering up to 100 W.

It supports:

• Four-pair power delivery
• Granular power requests (in 0.1 W increments)
• Cable loss compensation
• Bidirectional communication for power reallocation

This allows dynamic, safe, and efficient distribution of power across multiple high-demand PDs — a critical feature for smart buildings and industrial networks.

7. Real-World Example: LLDP in Action

Consider a Wi-Fi 6 access point connected to a PoE++ switch:

1. At startup, the PD is classified as Class 4, drawing 25.5 W.
2. After boot, it uses LLDP to request 31.2 W to power all radio chains.
3. The switch checks its power budget and grants the request.
4. If more devices connect later, LLDP allows the switch to reduce allocation dynamically.

This intelligent negotiation ensures:

• Stable operation of high-performance devices
• No overloading of switch power budget
• Efficient energy use across the network

8. LINK-PP Components Supporting LLDP-Enabled PoE Designs

Reliable LLDP-based communication requires stable signal integrity and robust current handling at the physical layer.
LINK-PP provides PoE RJ45 connectors with integrated magnetics optimized for IEEE 802.3at / bt compliance and LLDP-enabled systems.

Features:

• Integrated transformer & common-mode choke for LLDP signal clarity
• Supports 1.0A DC current per channel
• Low insertion loss and crosstalk
• Operating temperature: -40°C to +85°C

These components ensure that power negotiation packets (LLDP frames) remain clean and reliable, even under full power load.

9. Quick FAQ

Q1: Does every PoE device use LLDP?
Not all. LLDP is optional in PoE+ (802.3at) but mandatory in PoE++ (802.3bt) for advanced negotiation.

Q2: Can LLDP adjust power in real time?
Yes. LLDP allows continuous updates between PSE and PD, adapting power allocation as workloads change.

Q3: What happens if LLDP is disabled?
The system falls back to class-based power allocation, which is less flexible and may under- or over-power the PD.

10. Conclusion

LLDP brings intelligence and flexibility to Power over Ethernet systems.
By enabling dynamic communication between PSE and PD, it ensures each device receives just the right amount of power — no more, no less.

As networks scale and devices become more power-hungry, LLDP-based PoE negotiation is essential for optimizing energy use, maintaining reliability, and supporting next-generation devices.

With LINK-PP PoE RJ45 connectors, designers can ensure stable LLDP signaling, strong current endurance, and long-term network performance in every PoE application.