[great-potential]

Well, hate to tell you but you'll be limited 10Gb/s with these Intel cards because they require FEC.

[secure]

You mean 10 Gbit/s per link? Even on the 25 Gbit/s card?

Where can I read more about FEC (forward error correction?) and how that affects link speeds?

[great-potential]

Yes the transceivers recommended by Init7 (bidi-LR) do not support FEC and you'll be running in degraded mode (according to the controller datasheet), and I don't think this is something that can be achieved by coding of the transceivers.

[secure]

init7 requirements are at https://www.init7.net/en/support/router-information/

The specific 25G optic is https://www.flexoptix.net/en/p-b1625g-10-ad.html?co10426=972... and indeed does not support FEC.

The remote end is a Cisco C9500-48Y4C, on which one can turn off FEC.

On the Intel side, I found https://www.intel.com/content/dam/www/public/us/en/documents..., which mentions that while No-FEC might result in “poor link quality” (sure, that’s why people use FEC), it does not say anything about degraded mode or limits to 10G.

By controller datasheet, I take it you’re referring to https://www.mouser.cn/datasheet/2/612/710_series_datasheet_v...

That document contains a state machine diagram listing 25G AUI‐RS, 25G AUI‐FC and 25G AUI‐N (in order) before the fallback to 10G SFI.

Did you misread or am I missing something?

In either case, I’ll try the card. If it won’t link at 25G, I can get a different one. Any recommendations? :)

[secure]

The init7 CTO confirmed he’s running that same intel card and he’s getting 25 Gbit/s speeds: https://twitter.com/spale75/status/1414644121092952065

[great-potential]

Great news! tbh I just assumed from past exprience with Intel NICs that it will negotiate to lower speed without FEC.

Personnaly I'll probably go with some cheap mellanox card on ebay :)