Re: Standard MTU for SJFs and support for MEF 3.0 AI payloads

[Mikael Abrahamsson via NANOG <nanog () lists nanog org>]

On Mon, 9 Mar 2026, Carl Fredrik Lagerfeldt via NANOG wrote:

> Is 9530 a good value? What would you configure? (Am I simply 
> overthinking this?)

I did a very brief overview of this a few months back, and found several 
references to 9600.

https://business.comcast.com/support/article/ethernet/wavelength-service 
being one of them.

"Transport interfaces can support  frame sizes of up to 9600 MTU. "

Not sure what "frame size" is in this context exactly, if's calculated 
with 14 bytes of ethernet header or something else.

https://www2.arelion.com/ethernet-vs-waves-flyer , 1299 also say 9600 on 
DWDM (but it seems you're now saying you could support more?)

I don't think we generally can agree on a frame size in this industry, 
4470 and 9180 were the IP MTUs of classical old transport, but nothing say 
we should stay there.

So 9530 sounds as good as any. Just define it well so everybody knows 
what's up. I've tried asking several providers what their number means, if 
it's L2 or L3 MTU they're mentioning, and generally the 
provisioning/installation people don't know the difference so I've had to 
empirically test what it was between those two variants.

--
Mikael Abrahamsson    email: swmike () swm pp se
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