One question we get in our inbox regularly is how can you use a 40GbE (QSFP+) NIC with a 10GbE (SFP+) switch. Followers of STH will notice we stopped purchasing 10GbE (SFP+) network controllers in Q4 2016. As a result, we have started purchasing dozens of 40GbE (QSFP+) network adapters. One of the big drivers is cost as dual QSFP+ adapters are only around 100-150% more than dual SFP+ adapters for us. From time-to-time, in the DemoEval lab, we need to connect one of the machines onto our SFP+ switches.
Here is what you need for QSFP+ NIC to SFP+ switch
It turns out, there are adapters for converting a QSFP+ port to a SFP+ port. We tend to use Mellanox adapters since they are plentiful at around $35 each new (check Amazon
Using the adapter is very easy. You plug the larger QSFP+ adapter into the NIC and plug a smaller SFP+ into the adapter.
The benefit of this is that you can use existing SFP+ infrastructure without having to re-wire. When you swap to QSFP+ you can simply remove the adapter. For new machines in 2017 and forward, when we add additional NICs we plan to use these adapters regularly in infrastructure still on 10GbE switches.
What about a QSFP+ switch port to a SFP+ NIC?
This exact same method does work on most switches. We do not recommend using these QSFP+ to SFP+ converters on QSFP+ switches. The reason for this is that you can generally utilize either a QSFP+ to SFP+ DAC or a QSFP+ to 4x SFP+ breakout cable to achieve a similar result.
There are a few caveats to utilizing QSFP+ to SFP+ breakout cables. First, the switch ports must support this type of operation on the port you are trying to use it on. There are switches that do not support breakout cables. There are others that only support them on a limited set of ports. Second, these are generally messier to cable with since each SFP+ cable is the same length. Third, while there are optical breakouts, the copper breakout cables have limited DAC length runs.
Final Words
We hope this helps folks who may need to connect a QSFP+ NIC port to a SFP+ switch. The adapters are very easy to use and we have not run into cable coding issues thus far on our Arista, QCT and Mellanox switches. As 40GbE NICs continue to fall in price, having QSFP+ to SFP+ converters around is a cheap option to allow one to easily utilize existing SFP+ infrastructure.
 network adapter with a 10GbE (SFP+) switch. We show how you can easily do the conversion using Mellanox adapters){kind=link}
I want to connect two servers with QSFP+ directly without a switch in between. Are there crossover cables to do so?
Is there any way to get a Mellanox 40Gbps QSFP+ NIC to connect up to four SFP+ ports on a switch so the whole line rate is available?
Yes. Many switches support this using breakout cables.
Is there any concern for compatibility on the 10Gb SFP+ side of things?
David, that is a hard one to answer. We have tested with probably 20 different DACs/ optics and have not had issues. Of course, as soon as we say that others will come out and say they have found problems. Perhaps posting in for forum asking about the specific module may be helpful to you if you want to find someone else with the same NIC, adapter, SFP+ that you are thinking of.