I finally managed to order my Bluefield-2 CECOT DPUs but the provider could not offer cabling for purchase.
According to this NVidia post, QSFP ports are backward compatible; in fact, from QSFP-DD (400GB) all the way to QSFP+ (40G) are all backward compatible. However, the modulation used might be different. So, I am afraid now of purchasing a QSFP28 cable (using NRZ modulation) for the Bluefield-2 DPU having QSFP56 (PAM4 modulation).
Will it be compatible?
The second question is whether I should get a certain brand, or all these cables (should) strictly follow the standards and they don’t do some nasty tricks to do vendor-lockin? Like back in the days when certain mobile phones even though were using standard micro USB port, the pins were purposely swapped to disable charging it with other than the brand’s own charger. I know that this question is probably not a 100% sure Yes/No, as noone knows what random vendor might use; but any suggestions? For instance, can I buy QSFP cables from Mikrotik (originally sold for Mikrotik routers)?