NSX for vSphere allows trust of the Differentiated Service Code Point (DSCP) marking originally applied by a virtual machine, or explicitly modifying and setting the DSCP value at the logical switch level. In both cases, the DSCP value is propagated to the outer IP header of VXLAN encapsulated frames. This enables the external physical network to prioritize the traffic based on the DSCP setting on the external header. Both quality of service (QoS) and DSCP are good examples of how physical and virtual networking can work together under a common set of rules. Using both QoS and DSCP which are networking standards, allows network switches to prioritize certain network traffic over others, which in turn helps your critical workloads get the network priority required to meet business demands.

You can verify that the application traffic flowing through the physical network infrastructure is prioritized by using the following:

Traffic can be classified in different ways. In a Layer 2 frame, the 802.1q header contains the information for the class of service (CoS). The first 16 bits are always 0x8100, which means that the header contains a VLAN tag. The class of service is in the next 3 bits followed by a flag that indicates whether to fragment.

Layer 3 has a different field called DSCP that has 6 bits. The first three values typically match the first three CoS bits. At the boundary between Layers 2 and 3, the switch can take the CoS and other factors like the source or destination address and match that to a Layer 3 DSCP value. Because DSCP has more potential values, it can be more specific about the service that it is going to provide.