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I'm looking at a switch like this:

https://www.fs.com/products/29123.html

It has 48 10-GbE SFP+ ports and 6 40-GbE uplink ports. I would like to use some of these uplinks to cross-connect to another switch. In addition to that, however, I have a server with a 40-GbE capable NIC in it. Can this be connected directly to the 40-GbE port on the switch and "just work," or are these uplink ports special in that they must connect to uplink ports on another switch?

I know this used to be a thing for copper-based cabling before Auto-MDIX became commonplace, but it's not clear to me whether it's still an issue for modern SFP/QSFP-based connections.

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    Without knowing the server specs, one can't say for sure, but I would expect that the NIC was designed to connect to a switch (you could ask what else would it connect to?). – Ron Trunk Mar 6 at 15:15
  • The server has this Mellanox NIC in it. I was making the assumption that there would be a standard answer to this type of question, as it's just a "vanilla" Ethernet NIC. The inherent question is whether the uplink ports on a switch like this are functionally any different from the others. – Jason R Mar 6 at 15:18
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That switch uses (Q)SFP+ ports which are essentially PHYless. You need to either use matching PHY type transceivers on both the switch and the host ports (e.g. 10GBASE-SR or 40GBASE-SR4) or direct-attach cables that are compatible with both sides.

The Mellanox NIC has QSFP28 ports that should support QSFP+ modules as well - check the documentation.

Note that most switches or NICs only support correctly branded transceivers - directly from the vendor or 3rd party compatible ones.

Whether the switch connects to a host or to another switch generally doesn't matter, assuming the port functions are configured appropriately.

MDI/MDI-X/Auto MDI-X are a special function of twisted-pair ports. The reason is that TP cables are straight-through historically and the receiver/transmitter crossover happens inside one of the link ports.

(Q)SFP(+) ports are all the same, the crossover happens within the cable (fiber or DAC).

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    "Whether the switch connects to a host or to another switch generally doesn't matter" is the real answer, maybe should be on top? – aaaaaa Mar 6 at 23:49
  • @aaaaaa Well, it may in some rare instances (twisted pair without Auto MDI-X), so I wanted to make sure the scenario is clear. I do actually disable Auto MDI-X on downlink and edge ports, and leave it enabled only on uplink ports. – Zac67 Mar 7 at 21:04

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