An L2 sub-interface is a logical bridging endpoint associated with traffic on an interface distinguished by 802.1Q tags, where each <interface, 802.1Q tag> tuple is treated as a first-class bridging interface. 

Ingress policing provides the ability to monitor the data rates for a particular class of traffic and perform action

Many a times it helps to make sure the service, that is going to be provided to a customer, is really going to work as