Overview
The data link layer classification DLC for PoE provides more exact control over the power requirement between a PSE and PD. The DLC works in conjunction with the physical layer classification PLC and is mandatory for any Type-2 PD that requires more than 12.95 watts of input power.
DLC is defined as part of the IEEE 802.3at standard.
The power negotiation between a PSE and a PD can be implemented 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 repeatedly query the PD 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.