A facility for customizable routing in a modularized routing environment is provided, where the complexities of routing functionality are not exposed to an application program. An identifier associated with a routing policy of an application program is used to configure a route table and to obtain routing instructions from the route table that are associated with the identifier. Routing is performed according to a message routing protocol, such as the Diameter protocol, for example. An interface between the facility and the application program may be implemented with an API that provides a relatively simple interface.