In my experience, if you have a high-speed interconnect such as IB, you would do IPoIB. You would likely still have a "regular" Ethernet connection for management purposes, and yes that means both an IB switch and an Ethernet switch, but that switch doesn't have to be anything special. Any "real" traffic is routed over IB, everything is mounted via IB, etc. That's how the last two clusters I've worked with have been configured, and the next one will be the same (but will use Omnipath rather than IB). We likewise use BeeGFS.

These next comments are perhaps more likely to encounter differences of opinion, but I would say that sufficiently fast Ethernet is often "good enough" for most workloads (e.g., MPI). I'd wager that for all but the most demanding of workloads, it's entirely acceptable. You'll also save a bit of money, of course. HOWEVER, I do think there is, shall we say, an expectation from many researchers that any cluster worth its salt will have some kind of fast interconnect, even if at the scale of most on-prem work, you might be hard-pressed in real-world conditions to notice much of a difference. If you're running jobs that take weeks and hundreds of nodes, the time (and other) savings may add up, but if we're talking the difference between a job running on 5 nodes taking 48 hours vs. slightly less, then?? Your mileage may vary, as they say...

Warmest regards,

Jason