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:

• MDI power support

• PSE power Pair: The PSE power pair field contains an integer value as defined by the pethPsePortPowerPairs object in IETF RFC 3621.

• Power Class: The power class field contains an integer value as defined by the pethPsePortPowerClassifications object in IETF RFC 3621.

• Type/Source/Priority

• PD Requested

• PSE allocated

 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

• All PoE devices support PoE over LLDP power negotiation function, except for S5810-48TS-P, S3410L-24TF-P and S5860-24XB-U.
• Enable LLDP function and PoE function before using PoE over LLDP power negotiation.

• When an expansion module is connected to the IP phone, in order to provide the required power to the expansion module, user should perform the following two operations to re-negotiate the PoE power supply.

  1).  Enable the PoE over LLDP power negotiation function on the interface by using set poe interface <all | interface-name> lldp-negotiation <true | false> command.

  2).  Disable and re-enable PoE function of the interface by using set poe interface {<port-id> | all} enable <true | false> command.

Procedure

• Use the following command to disable or enable the PoE over LLDP power negotiation function.

       set poe interface <all | interface-name> lldp-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:

• An IP phone is connected to the Ge-1/1/1 port of the switch and an expansion module is connected to the IP phone.

• Enable PoE on port Ge-1/1/1 of the switch.

• When using PoE to power the IP phone, it also provides power for the expansion module which is connected to the IP phone. In this case, you need to enable the PoE over LLDP power negotiation function on the interface to assign the power priority and power value to the IP phone through LLDP power negotiation.

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

Step4         Commit the configuration.

admin@Xorplus# commit