Ok good to know that you must force 56Gb.I had to set 56gbe from web gui section first even for mellanox cables. Default mode was 40gbe. I got my 3m cables off ebay around 45$/each. Dell DAC was around 35 for 7m.
MC2207130 is passive copper cable,that will go 56gbe infiniband and 40gbe ethernet,it might be more latency in 40gbe ethernet mode than active fiber cable,its important in storage fabrics.
I upgraded this weekend to 3.6.8010 and needed two days to check that they improved their security level. For web gui you need https now (see release notes to 3.6.8010).can someone help me too ; i have just updated to 3.6.8010 but i am lost in what to do .