How PoE interacts LLDP and LLDP-MED?

Question

I need to research 802.3at PoE PSE capable equipment for a project in my company's network, but I have some knowledge gaps regarding PoE operation and its reliance on LLDP.

I took a look over 802.3-2018 and I found this:

The TLV shown in Figure 79–3 is a revision of the legacy Power via MDI TLV originally defined in IEEE Std 802.1AB-2009 Annex F.3. The legacy TLV had only the first three fields of the TLV shown in the figure. These three fields enable discovery and advertisement of MDI power support capabilities. The newly added fields provide Data Link Layer classification capabilities. The revised TLV can be used by the PSE only when it is supplying power to a PI encompassed within an MDI and by the PD only when it is drawing power from the PI. Power entities may continue to use the legacy TLV prior to supplying/drawing power to/from the PI. If the power entity implements Data Link Layer classification, it shall use the Power via MDI TLV shown in Figure 79–3 after the PI has been powered.

As I understand, the mandatory fields for my 802.3at-powered network are MDI power support, PsE power pair and Power class. The other tree fields are needed only for poe l2 classification, which I understand that is not mandatory, only optional. What I don't understand is that there seems to be a PoE LLDP-MED TLV and, according to the source (including vendor manuals and documentation):

• The LLDP PoE TLV follows either the legacy format (the first three fields) or the updated one (all fields) and the device also supports an additional LLDP-MED PoE TLV extension with additional info.
• The PoE LLDP TLV = the legacy format and the LLDP-MED extension = the other three fields (TLV information string continued from the standard diagram).
• The PoE TLV is exclusive to LLDP-MED extension of LLDP.

This is really confusing to me and what I would like to clarify is:

• Is PoE TLV exclusive to LLDP-MED or not?
• What fields of PoE TLV are mandatory and which are optional for a 802.3at implementation?
• If LLDP PoE TLV includes all the fields from above (as the standard indicates), what additional fields does LLDP-MED brings to PoE?

Answer 1

PoE power allocation can happen on the physical layer by signature resistance, or on the data link layer by LLDP TLV (PD→PSE). Which one takes precedence is a matter of configuration in your switches/PoE injectors (PSE).

LLDP can be used for lots of other purposes. LLDP-MED is a mechanism that is mainly used to pass information to an IP phone, like VLAN, DSCP, room, street address (switch→phone). I may also be used as an alternative to native PoE TLVs but may be completely unrelated as well.

Is PoE TLV exclusive to LLDP-MED or not?

No - there's native LLDP PoE TLV and there's LLDP-MED, alternatively.

What fields of PoE TLV are mandatory and which are optional for a 802.3at implementation?

LLDP TLV are only mandatory if your PSE is configured to require them. You can use L1 allocation exclusively.

If LLDP PoE TLV includes all the fields from above (as the standard indicates), what additional fields does LLDP-MED brings to PoE?

Basically, LLDP MED allows for more granular power allocation (by device/phone type/model) than native PoE TLV (by power class alone). But it all depends on the devices you use and their configuration.