I do know that not all of them support breakout. The older cards don’t support it card-side I’m pretty sure (adapter works for 2/3-Pro, can vouch for that as I own those models, but of course you’re leaving 30Gbit/s on the table that way). Some Infiniband switches with Ethernet support do support breakout, but that requires an Infiniband switch that supports Ethernet, which is an expensive license (could get really lucky and score one with a license already applied on, say, Ebay, of course)
If you run them in Ethernet mode they just work like any other Ethernet card.
Are you are asking whether connecting a ConnectX-2 with a QSFP+ cable to a 40Gbase Ethernet switch would work? I imagine the way to test this (without getting a switch) would be to direct-connect a 40Gbe capable ConnectX-model to a 10Gbe-only capable ConnectX-2 with a QSFP+ cable (all in Ethernet mode, of course). I would expect it to link up just fine at 10Gbe speeds, but never tested it.
Btw, note that, in case you were not aware, not all ConnectX-3 models can push 40Gbe, you need one of the FDR capable cards (QDR doesn’t have enough bandwidth for 40Gbe).
Also a note, Connect X2 cards are slowly losing support even in linux. I would not suggest getting anything older than x3 at this point unless its only to mess around and you wont ever need anything to actually run on it.
Some enterprise oriented distros might, but I rather doubt Linux (as in: the kernel) is dropping support for these anytime soon, unless they become a massive maintenance burden. They’re still perfectly usable, and quite popular, despite being “old”.
But indeed, something to be aware of when wanting to run enterprise distros at home, those often trim “unsupported” (by the vendor) hardware pretty aggressively.
Thanks all for the replies, yes my main question was would a 40gbe capable card connect at 10gbe with the older card and the same for a switch not using a breakout cable but the QSFP+ cables I already have.
For the ConnectX-3 card I was looking at the MCX354A-FCBT am I wrong in thinking that card can/does use a QSFP+ cable to connect?
Regarding using them in InfiniBand my knowledge of that is almost zero but my understanding is as one of my os’s is windows this wasn’t an option, if I am wrong in this please let me know as that would be great.
A big part of my thinking in going for the ConnectX-3 cards is they are now a lot cheaper than they were and they support pcie 3.0.
That’s an FDR capable card, so 40GbE should work according to the spec sheet. Just mentioned it because there are QDR ConnectX-3 variants too (like the MCX354A-QCBT) and those will only link at 10Gbit in Ethernet mode.