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.
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?