Cisco Nexus Routing Switching – Control Plane Terminology Explained

From the Cisco NX-OS 5.1(3)N1(1) release and later releases, each Cisco Nexus 5500 Series device can manage and support up to 24 FEXs without Layer 3. With Layer 3, the number of FEXs supported per Cisco Nexus 5500 Series device is 8. With Enhanced vPC and a dual-homed FEX topology each FEX is managed by both Cisco Nexus 5000 Series devices. As a result, one pair of Cisco Nexus 5500 Series devices can support up to 24 FEXs and 16 FEXs for Layer 2 and Layer 3.
There are differences in scalability between the straight-through topology, the dual-homed FEX topology, and the Enhanced vPC topology. In the straight through topology, only one Cisco Nexus 5000 Series device manages each FEX and a pair of Cisco Nexus 5500 Series devices manage up to 48 FEXs

Because the total number of FEXs that are supported by a pair of Cisco Nexus 5000 Series devices is different between these two topologies, the FEX straight-through design with more than 24 FEXs per one pair of Cisco Nexus 5000 Series devices cannot migrate to Enhanced vPC topology.

The configurations are as follows:

•Total number of host vPC — With Enhanced vPC, each FEX port can be part of a host vPC. The host vPC does not consume port channel resources on the parent Cisco Nexus 5000 Series device.

•Total number of ports per host vPC — The total number of ports that can be assigned to each host vPC differs with each FEX model.

please may you check this link Using Enhanced vPC

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.