Legacy interface gives the flexibility to create sub-interfaces which is also known as logical inteface. Every logical interface consumes one IDB which has the limitation of scalability factor of vlans in the box. But the Metro Ethernet architecture which is EVC comes up with new thing which is service instance. It gives the many more functions with addition to the legacy logical interface. Service Instance could be considered as a service which can use a single port with the combination of layer 2 and layer 3 ports. The data entering in physical ports matches the service instance rules and filters the data according to the rules. We can create different type of service instances and bind all the service instances on the single physical port which gives the flexibility to service provider in terms of scalability. The another advantage of using service instance is that one can bridge the different domains but the same functionality could not be used by the sub-interfaces.
Different Type of Service Instances:-
1. Single Dot1q tag matching
2. Double tag matching
3. Untagged or native packets matching