POE negotiation between IEEE PD and the POE Switch?

Question

I know that there happens a negotiation between the IEEE PD devices and directly connected POE Switch through CDP or LLDP protocols. But if for some unknown reasons, if the CDP/LLDP is disabled on the IEEE PD. How much power does the switch allocate for that particular powered device if the negotiation fails? What is the importance of this negotiation? How does this negotiation change when the POE+ connected to the Switch port?

Really appreciate if you can help me understand these concepts well.

Answer 1

IEEE PoE commonly uses MDI negotiation first (PLC = physical layer classification) - to enable a PD to power up. The MDI uses signature resistors.

Often, MDI PoE is overridden by LLDP (MED) TLV (DLC = data link layer classification) once the device is powered up.

Whether MDI or LLDP have priority can be configured on most switches. Which current classes are detected can be displayed by e.g. using show power-over-ethernet [port] on HPE Provision switches.

Many switches can also allocate power by simply monitoring the load. This may be necessary for pre-af devices.

Power negotiation and allocation are important in case the PSE reaches its capacity. There's always a clear decision which ports are shut down, based on LLDP-MED priority, port priority, power class, ...

Rare PDs can even reduce their power draw when power allocation is withdrawn by the PSE (e.g. a phone shuts off its display but stays functional).