Introduction

Power over Ethernet (PoE) enables electrical power to be carried over Ethernet cables, thus simplifying the deployment of networking equipment by eliminating the need for a power cord. Enabling it requires users to familiarize themselves with various PoE commands.

On this page, you will learn about one such PoE command, PoE over Link Layer Discovery Protocol (LLDP) power negotiation. This function allows a network switch, including an open white box switch, to assign the power priority value using LLDP to negotiate a power value, as opposed to using the power priority and value configured on the switch interface. The power negotiation process is described in detail, as is the switch configuration process.

Power via MDI TLV

IEEE 802.3 defines Power via MDI TLV to allow network management to advertise and discover power capability between LLDP-MED Endpoint and Network Connectivity Devices by sending LLDP negotiation packets.

The Power via MDI TLV is composed of 2-byte packet header and 12-byte TLV information field, as shown in Figure 1.

Figure 1. Power via MDI TLV packet format


The fields of Power via MDI TLV are described as follows:

Field                       

Functions

Description

0

Port type

1: PSE-side port

0: PD-side port

1

Whether the PSE supports MDI power supply.

1: indicates that the PSE supports MDI power supply.

0: indicates that the PSE does not support MDI power supply.

2

MDI power supply status of the PSE.

1: enabled

0: disabled

3

Whether the PSE can control the line pair.

1: indicates that the PSE can control the line pair.

0: indicates that the PSE cannot control the line pair.

4-7

Reserved.

-

 

Field

Functions

Description

Priority

Power supply priority of an interface.

11: indicates the lowest priority.

10: indicates the secondary highest priority.

01: indicates the highest priority.

NOTE:

This field contains four bits. The two left-most bits are reserved for the system.

Source

 

 

 

 

Power supply source.

PD:

  • 11: indicates the PSE and local source.
  • 10: indicates the local.
  • 01: indicates the PSE.

PSE:

  • 11: indicates to be reserved.
  • 10: indicates the backup power supply.
  • 01: indicates the primary power supply.

Type

 

 

 

 

Power supply type.

01: indicates the PD that supports IEEE 802.3at.

00: indicates the PSE that supports IEEE 802.3at.

11: reserved.

10: reserved.

Field

Functions

Value/Meaning

PD requested

PD requested power value

  • Power=0.1*(decimal value of bits) Watts. The value is an integer that ranges from 1 to 255. For example, if the value of the field is 255, the exchange power is 25.5 W.
  • When the PoE power is sufficient, the values of PD Requested Power value and PSE allocated Power value are the same.

Field

Functions

Value/Meaning

PSE allocated

PSE allocated power value

  • Power=0.1*(decimal value of bits) Watts. The value is an integer that ranges from 1 to 255. For example, if the value of the field is 255, the exchange power is 25.5 W.
  • When the PoE power is sufficient, the values of PD Requested Power value and PSE allocated Power value are the same.

 PoE over LLDP Power Negotiation Mechanism

If PoE over LLDP power negotiation function is enabled on PSE, the PSE and the PD will periodically send LLDP packets to each other to perform LLDP power negotiation once the PD device is detected by the PSE. PoE over LLDP power negotiation starts when PSE receives LLDP packets from PD.

PoE over LLDP power negotiation message mechanism is as follows:

1.    PD sends an LLDP packet to the PSE carries Power via MDI TLV, which contains the power supply priority of an interface and the PD requested power value.

2.    PSE replies with an LLDP packet to the PD device. After receiving the LLDP packet from the PD device, the PSE allocates the corresponding power to the PD device and encapsulates it into the PSE allocated power value field in the Power via MDI TLV according to the PD requested value and its own power capabilities.

Configuring PoE over LLDP Power Negotiation

Configuration Notes

Procedure

       set poe interface <all | interface-namelldp-negotiation <true | false>

       By default, the function is enabled. To make the configuration take effect,  user need to disable and re-enable PoE function of the interface by using set poe interface {<port-id> | all} enable <true | false> command.

Configuration Example

Networking Requirements

The networking requirements are as follows:


Procedure

Step1         Enable LLDP function.

admin@Xorplus# set protocols lldp enable true

Step2         Enable PoE function on interface Ge-1/1/1.

admin@XorPlus# set poe interface ge-1/1/1 enable true

Step3         Enable the PoE over LLDP power negotiation function on interface Ge-1/1/1.

admin@Xorplus# set poe interface ge-1/1/1 lldp-negotiation true


When an expansion module is connected to the IP phone, in order to provide the required power to the expansion module, user should disable and re-enable PoE function of the interface by using set poe interface {<port-id> | all} enable <true | false> command after enabled the PoE over LLDP power negotiation function.


Step4         Commit the configuration.

admin@Xorplus# commit