In a vSphere distributed switch 5.5 and later, by using the traffic filtering and marking policy, you can protect the virtual network from unwanted traffic and security attacks or apply a QoS tag to a certain type of traffic.
The traffic filtering and marking policy represents an ordered set of network traffic rules for security and for QoS tagging of the data flow through the ports of a distributed switch. In general, a rule consists of a qualifier for traffic, and of an action for restricting or prioritizing the matching traffic.
The vSphere distributed switch applies rules on traffic at different places in the data stream. The distributed switch applies traffic filter rules on the data path between the virtual machine network adapter and distributed port, or between the uplink port and physical network adapter for rules on uplinks.