I've been repeating that 25gbe is backwards compatible with 10gbe for years now. Hell, I probably have ~100 links now where one side is a 25gbe switch/nic and the other is 10gbe.
However, recently I've been burned repeatedly. I have 5 nics and 2 switches that refuse to talk at anything but full 25gbe. They recognize 10gbe transceivers, but don't down-negotiate to them. Hard setting the speed doesn't seem to have an effect: I still can't get them to bring up a link.
Switches are Dell S5224f-ON units, and the problem persists across OS10, pica8 and SONiC nightlies.
The NICs are Marvell / Qlogic Fastlinq ql41212h, also of Dell origin. On this side we've been through multiple OS's without changing the behavior.
Anybody seen anything like this? Am I missing something really stupid, or have I just gotten lucky with a pile of gear that doesn't behave as every other unit on the planet does?
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