PCIe 3.0 x16 card in x16 physical / x8 electrical slot?

I’m new to Mellanox/NVIDIA and am interested in MCX515A-CCAT. As a 100 GbE card, I realize this card would need PCIe 3.0 x16 to reach its full 100 GbE speed. What happens if I put this card in a PCIe 3.0 x16 physical / x8 electrical slot? Since x8 slots max out at 64 Gb/s in one direction, will this card try to reach 100 GbE speeds or will it shift down into 50 GbE mode?

Thank you!


The MCX515A-CCAT was designed for a full PCIe Gen.3 x16 connection as listed in its User Manual specifications. Utilizing it with a narrower PCIe width would be an unsupported configuration.

In cases where the PCIe width is narrowed, the adapter’s link speed will not be limited and will still link at the 100GbE speed, but you will not achieve 100GbE performance as the performance will ultimately be bottlenecked by the PCIe throughput.

I would recommend reviewing the following community article for more information on determining your systems’ PCIe capabilities, as well as how to calculate their limitations at the PCIe level:


More information on the ConnectX-5 can also be found here:



-Nvidia Network Support