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How does PoE device classification work?

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TitleHow does PoE device classification work?
Question
How does a PoE switch classify an 802.3af or 802.3at PoE device?
Environment
Power over Ethernet
Answer
Definitions of terms used in this article:
  • Power Sourcing Equipment (PSE) - any device that is capable of providing power over ethernet
  • Powered Device (PD) - any device capable of receiving power over ethernet from a PSE
  • Type 1 PSE or PD - an 802.3af compliant PSE or PD
  • Type 2 PSE or PD - an 802.3at (PoE+) compliant PSE or PD

When the PSE detects a connected PD, it will send a short voltage pulse to determine how much power the PD needs. Depending on how much current is drawn during this pulse, the PSE will classify the PD into one of classes 0-4 (or 0-3 for type 1 PSEs and PDs).

If a type 2 PSE classifies the connected PD into class 4 (PoE+ power), a second handshake is required. This is both to allow the PD to know that full 802.3at power is available, as well as to allow the PSE to confirm that a type 2 PD is actually connected to the port for safety reasons. Prior to this second handshake taking place, the PD and PSE will restrict power to 802.3af limits.

This second handshake can take one of two forms: a hardware-based Two-Event Classification or a software-based LLDP classification. Per the 802.3at standard, PSEs must support only one of these mechanisms. However, PDs must support both classification methods.

In the hardware-based two-event classification, the initial voltage pulse classification event is repeated. Once the PD passes the second classification event as a class 4 PD, it will be provided full class 4 802.3at power. This method was initially intended to be used by mid-span PoE injectors, as they generally will not support LLDP required for software-based classification.

For the software-based LLDP classification, LLDP must be enabled on both the PSE and PD, with the 802.3at TLVs enabled. This will allow the PSE and PD to negotiate power requirements via software, rather than the second classification pulse used in the hardware based method. This also allows power requirements to be dynamically negotiated, allowing a PSE's power budget to be better managed.



 
Additional notes
Note that the original generation Summit X440 and Summit X460 support 802.3at PoE+, but do not support hardware-based two event classification. Because of this, any type 2 PDs connected to these switches must be configured to allow LLDP classification. LLDP must also be enabled on the ports connected to the PD with the 802.3at TLVs enabled.

See the following article for further details on configuring this in EXOS:
How to enable LLDP (Layer 2) classification

All second generation PoE Summit switches support both software-based and hardware-based 802.3at classification.

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