Power over Ethernet/Power over Ethernet Plus allocation using Link Layer Discovery Protocol (LLDP) supports Ethernet switches, which do not support hardware-level power negotiation. With this feature, these switches support IEEE-based PoE and play the role of power sourcing equipment (PSE).
The devices that are powered using PoE, such as IP Phone and Video Surveillance Cameras, are classified as Powered Devices (PD). The maximum allowed continuous output power per cable for each standard is:
IEEE 802.3af: 15.4 W
IEEE 802.3at: 25.5 W
IEEE 802.3bt Type 3: 60 W
IEEE 802.3bt Type 4: 90 W
The negotiation of actual power supply and demand between a PSE and a PD can be executed at either the physical layer or at the data link layer. After the link is established at the physical layer, the PSE can use the IEEE 802.1AB LLDP protocol to repeatedly query the PD to discover its power needs. Communication using LLDP allows for a finer control of power allocation, making it possible for the PSE to dynamically supply the exact power levels needed by individual PDs, and globally for all PDs that are attached. Using LLDP is optional for the PSE, however, it is mandatory for a Type 2 PD that requires more than 12.95 watts of power.
Important
LLDP supports PoE discovery and power allocation because some switches do not support hardware-level power negotiation. This allows Type 2 PDs such PTZ (pan-tilt-zoom) Video Surveillance Cameras to be fully functional when connected to one of these switches. This functionality is enabled by default and is not configurable.
Note
Some switches feature a hardware design that supports hardware-level detection. Therefore, they do not require LLDP.