PoE/PoE+ allocation using LLDP

LLDP with PoE

When using PoE, enabling poe-lldp-detect allows automatic power configuration if the link partner supports PoE. When LLDP is enabled, the information about the power usage of the PD is available, and the switch can then comply with or ignore this information. You can configure PoE on each port according to the PD (IP phone, wireless device, and so on) specified in the LLDP field. The default configuration is for PoE information to be ignored if detected through LLDP.


	NOTE: Detecting PoE information via LLDP affects only power delivery; it does not affect normal Ethernet connectivity.

LLDP with PoE+

PoE+ with LLDP Overview

The DLC for PoE provides more exact control over the power requirement between a PSE and PD. The DLC works in conjunction with the PLC and is mandatory for any Type-2 PD that requires more than 12.95 watts of input power.


	NOTE: DLC is defined as part of the IEEE 802.3at standard.

You can implement the power negotiation between a PSE and a PD at the physical layer or at the data link layer. After the link is powered at the physical layer, the PSE can use LLDP to query the PD repeatedly to discover the power needs of the PD. Communication over the data link layer allows finer control of power allotment, which makes it possible for the PSE to supply dynamically the power levels needed by the PD. Using LLDP is optional for the PSE but mandatory for a Type 2 PD that requires more than 12.95 watts of power.

If the power needed by the PD is not available, that port is shut off.

PoE allocation

There are two ways LLDP can negotiate power with a PD:

Using LLDP MED TLVs

Disabled by default. Can be enabled using the

int <PORT-LIST> PoE-lldp-detect [ enabled | disabled ]

command, as shown below.

LLDP MED TLVs sent by the PD are used to negotiate power only if the LLDP PoE+ TLV is disabled or inactive; if the LLDP PoE+ TLV is sent as well (not likely), the LLDP MED TLV is ignored.
Using LLDP PoE+ TLVs

Enabled by default. The LLDP PoE+ TLV is always advertised unless it has been disabled (enable it by using the lldp config <PORT-LIST> dot3TlvEnable poeplus_config command.)

It always takes precedence over the LLDP MED TLV.

Enabling PoE-lldp-detect allows the data link layer to be used for power negotiation.When a PD requests power on a PoE port, LLDP interacts with PoE to see if there is enough power to fulfill the request. Power is set at the level requested. If the PD goes into power-saving mode, the power supplied is reduced; if the need for power increases, the amount supplied is increased. PoE and LLDP interact to meet the current power demands.

Operation Note

The advertisement of power with TLVs for LLDP PoE+ is enabled by default. If LLDP is disabled at runtime and a PD is using PoE+ power that has been negotiated through LLDP, there will be a temporary power drop. The port will begin using PoE+ power through the PLC. This event is recorded in the event log. An example message would look like the following:

W 08/04/10 13:35:50 02768 ports: Port A1 PoE power dropped. Exceeded physical classification for a PoE Type1 device (LLDP process disabled)

When LLDP is enabled again, it causes a temporary power drop. This event is also recorded in the event log. An example message looks like the following:

W 08/04/10 13:36:31 02771 ports: Port A1 PoE power dropped. Exceeded physical classification due to change in classification type (LLDP process enabled)

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