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The use of 802.1 LLDP or 802.3 OAM for power negotiation



All,

I apologize for the crossposting of this to the wider 802.1 and 802.3 
groups. If it leads to long detailed discussions that are not of 
interest to the broad membership then I will take it to the 802.3at 
reflector. However, the issue is beyond the ken of the 
power-over-Ethernet experts and needs some input from standards and 
protocol geeks, to whit:

The 802.3at Task Force would like to define a mechanism for negotiating 
changes in power state. I have proposed a simple state machine to handle 
the request/acknowledge mechanism but the method of communicating the 
states and the power objects is causing some controversy.

Proposal 1.

Extend the current definition of LLD-MED for power objects to include 
the state information and extended objects as defined by 802.3at.

Advantages: Some (but not all) inline power systems already require the 
use of LLDP for power information in a stateless manner.
Disadvantages: LLDP has been defined as a stateless protocol, its use 
for this stateful mechanism violates a philosophical rule (potentially 
opening the door for abuse...). Also, changing the definition for LLDP 
in 802.1AB (10.2.4) would require opening a project in 802.1 to run 
parallel to 802.3at.

Proposal 2.

Use a variation of the 802.3ah OAM protocol to transport the state 
information and power objects. This would create a new slow protocol, 
OAM subtype, code - as defined in 802.3 Clause 57.

Advantages: 802.3ah OAM is specifically designed for handling the 
MAC/PHY management of a single link segment. The definition is contained 
within 802.3 and would create no confusion with currently defined 
stateless power management (e.g. TR41 & LLDP-MED).
Disadvantages: The effort may be seen as "re-inventing the wheel" or may 
be taken as a snub to 802.1. Some devices might be required to support 
both the 802.1 and the 802.3 protocols.

Discussion.

In my view, the definition in 802.1AB does not preclude its use for 
transporting state information - as long as the state machine is clearly 
defined outside the standard. I believe that we could define the power 
mode control state machine in 802.3 (Clause 33) and include an annex 
describing the use of 802.1 LLDP to carry the state information and the 
extended power objects. No project in 802.1 should be required.

On the other hand, if there is a philosophical barrier to using LLDP for 
this application then the additional burden of supporting 802.3ah will 
be relatively small for a small number of devices for a short time. A 
device that is sufficiently complex to support a layer 2 management 
protocol can generally support 2 similar protocols with little extra 
complexity. Furthermore, most of the devices that might require both 
LLDP and OAM would also be required to support SNMP - at a greater level 
of complexity.

Hugh.


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