In general, this disclosure describes a programmable network platform for dynamically programming a cloud exchange to provide a layer three (L3) routing instance as a service to customers of the cloud exchange. In one example, a cloud exchange comprises an L3 network located within a data center and configured with an L3 routing instance for an enterprise; and for the L3 routing instance, respective first and second attachment circuits for first and second cloud service provider networks co-located within the data center, wherein the L3 routing instance stores a route to a subnet of the second cloud service provider network to cause the L3 routing instance to forward packets, received from the first cloud service provider network via the first attachment circuit, to the second cloud service provider network via the second attachment circuit.