Are SFP+ "Ethernet" direct attach cables compatible with Fibre Channel?

Question

I'm trying to find some short 8G SFP+ Fibre Channel direct attach cables but they're rare and expensive, can a passive 10G SFP+ direct attach cable marketed for Ethernet be used between two 8G SFP+ Fibre Channel ports?

The connection is between a Dell/EMC VNX5300 to an 8G FC port on a Cisco Nexus N55-M8P8FP Fibre Channel expansion module in a 5548UP Ethernet and FCoE switch.

The answer is yes on an electrical level but from what I can tell there's an EEPROM chip in each pluggable that has metadata in it that hardware uses to determine if the cable is compatible or not, I'm worried that might include some kind of distinction between cables designed for "Ethernet" and cables designed for stuff like Fibre Channel or InfiniBand that hardware from certain vendors will pick up and block me from using the cable.

Does anyone have any experience mixing direct attach cables like this on Dell/EMC and Cisco Nexus hardware?

If it's not possible then I'll just buy a real 8G Fibre Channel direct attach cable and dump the contents of it's EEPROM and flash that onto the EEPROM of an SFP+ Ethernet cable and that should fix it.

Answer 1

In theory, yes. The base standard SFF-8431 for SFP+ (up to 10 Gbit/s nominally and 11.1 GBd physically) is the same for Ethernet and Fibre Channel.

You could use any 10G SFP+ DAC with 8GFC as well - if you get the host node (switch/NIC/HBA) to accept it. (Q)SFP/+/28 modules identify to their node by contents of an EEPROM that also contains a compatibility matrix.

If that matrix doesn't explicitly list 8GFC, the module is likely rejected. On some devices you might be able to force acceptance by explicit configuration, but rarely so. Of course, reprogramming the EEPROM would solve the problem with most vendors - some vendors use additional identification to prevent simple circumvention of the lock-in mechanism.