On Fri, 22 Jul 2022 14:23:18 +0100
"Russell King (Oracle)" [off-list ref] wrote:

On Fri, Jul 22, 2022 at 02:59:36PM +0200, Marek Behún wrote:

quoted

  The 2.5GBASE-X PCS does not support Clause 37 Auto-Negotiation.
  Hence, the 1000BASE-X PCS is expected to have its Clause 37
  Auto-Negotiation functionality disabled so that the /C/ ordered set
  will not be transmitted. If a 2.5GBASE-X PCS receives /C/ ordered
  set, then undefined behavior may occur.
  ...  

The reason that's probably stated is because there hasn't been any
standardisation of it, different implementations behave differently,
so they can't define a standard behaviour without breaking what's
already out there.

I think they are also considering clause 73 AN, which is supported with
1000base-KX (if I am not mistaken). Maybe the intention was to use
clause 73 with >1G speeds, and so they've forbid clause 37 on
2500base-x.