We are working on Stratix10 Development Kit. We implemented Intel E-Tile Hard IP based project. We connected this board with Mellanox board through 100g Twinax Copper cable provided with the FPGA board and the link is coming up. But when we used another 100G cable(both optical and copper of mellanox), link is not coming up with the FPGA.
Based on your issue description, looks like you have isolated the issue to be cable dependent, right ?
Most likely you are dealing with signal integrity issue here. You may want to check the difference between these 2 cables.
You are running high speed testing here which means SI design is critical design consideration to evaluated properly.
If Intel ship you bad cable then you can make request to complaint for replacement but now my understanding is the other way around. Other brand cable is not working here.
Thanks for the isolation test. This proven your cable is not broken. Then it could be due to cable performance issue here.
If your overall system transmission link doesn't have much margin left then higher insertion loss/return loss could eat up the margin and caused link down.