The system qos is a type of MQC target. You use a service policy to associate a policy map with the system qos target. A system qos policy applies to all interfaces on the switch unless a specific interface has an overriding service-policy configuration. The system qos policies are used to define system classes, the classes of traffic across the entire switch, and their attributes. To ensure QoS consistency (and for ease of configuration), the device distributes the system class parameter values to all its attached network adapters using the Data Center Bridging Exchange (DCBX) protocol.
If service policies are configured at the interface level, the interface-level policy always takes precedence over system class configuration or defaults.
On the Cisco Nexus device, a system class is uniquely identified by a qos-group value. A total of six system classes are supported. Two of the six system classes are defaults and are always present on the device. Up to four additional system classes can be created by the administrator.