Getting optical transceivers to work at higher signaling speeds (40G, 100G, 400G, 800G...) is hard, as even the the variation in the lengths of the electrical traces on the host's PCB come into play. Third-party optics vendors rarely, if ever, qualify their products by installing them in actual switches/routers and running performance/EMI/mechanical/environmental/etc tests against specific versions of host software. That means that their customers who bought their lesser-priced products on the good faith that all optics claiming to be MSA-compliant are the same, end up doing the testing for them. This arrangement works as long as the 3rd-party optics vendor is then willing to troubleshoot with the customer to analyze dumps of byte codes to arrive at RCA when there is a functional or performance issue. If the optics vendor is unwilling to troubleshoot their product, then the customer is faced with either replacing the optic, or replacing the switch/router.
But why would the "identical" transceiver from the same OEM that Cisco buys from function differently than the Cisco branded version? Because they are not necessarily identical, even though they may have the same OEM base part number. Cisco qualifies the OEM part by testing it in their products. When Cisco finds issues (I say "when", not "if", because issues are frequently found), they ask the OEM to make tweaks to the EEPROM (or mechanicals, or whatever). This then creates a new version of the base part specifically for Cisco. The OEM may, or may not, end up subsequently selling that same version on the open market that has the Cisco tweaks; a reason they may not sell it is because the production cost is a little higher, the production yield is lower, or the contract with Cisco has terminated and the OEM no longer manufactures the Cisco version. Another scenario is when, subsequent to the Cisco version, the OEM makes a component change to the base part for their own reasons, independent of Cisco. Cisco might then reject this change for their purchases if it does not pass qualification tests, while the OEM proceeds with their new version sales on the open market. These types of divergences are the primary reason why Cisco-branded and direct-from-OEM optics with the same base part number can have different behaviors in Cisco products.
Disclaimers: I am long in CSCO. Bad answers are my own fault as they are not AI generated.
