This paper presents the MRCP (Modular Routing Control Platform), a routing control architecture that provides complete control and visibility of interdomain routing in a single AS. We propose a set of principles for making interdomain routing control more flexible and extensible, and show how the design of the MRCP adheres to the principles and enables new functionalities and services. Categories and Subject Descriptors C.2.3 [Network Operations]: Network Management General Terms Routing, Design, Control, Management, BGP Keywords Modularity, Flexibility, Extensibility