Dear friend, I also conducted these tests with 2 switches crs326, with 2 switches src354, with and without MLAG and got the same result, 40G works with a direct connection between network cards. Mikrotik does not work with Mellanox cards at 40G speed, only 10G+ on cannal and work only 1 cannel. I test in on Mellanox ConnectX3 Pro.