Cisco Nexus 3048 – Enabling Flow Control

There is no way to do what you want to do. VDCs virtualize control plane and data plane in protected memory so that each is an independent virtual device, just like independent physical devices. You can, of course, ask Cisco to consider adding this feature in a future NX-OS release.

From Cisco Nexus 7000 Series NX-OS Virtual Device Context Configuration Guide:

Communication Between VDCs

The Cisco NX-OS software does not support direct communication between VDCs on a single physical device. You must make a physical connection from a port allocated to one VDC to a port allocated to the other VDC to allow the VDCs to communicate.

If the clasification isn't available on the switch model, then you need to look at something like ACL-based QoS classification (Layers 2, 3, and 4) so that you can mark what traffic, by IP address range for example and tell the hardware how you want to prioritise the traffic with mappings.

CoS is for traffic that is layer 2 only. If you need to prioritise traffic that is going to get routed, then you would use DSCP which is layer 3.

So CoS will put the marking on an Ethernet frame header and DSCP would do it within the space on a IP header. If the traffic is getting routed, the Ethernet frame get's stripped so you would loose your CoS marking and would rely on DSCP within the IP header.

You achieve more granuality with DSCP compared to CoS as you can set more options because of the difference in bit size since CoS uses only 3 bits to set markings while DSCP uses 6 bits.

All in all, I would advise you seperate your ISCSI network from your core infrastrucure and then you wouldn't even need to worry about QoS.