1. A network control system for generating physical control plane data for managing first and second managed forwarding elements that implement forwarding operations associated with a first logical datapath set, the system comprising: a) a first controller instance for converting logical control plane data for the first logical datapath set to universal physical control plane (UPCP) data; and b) a second controller instance for converting UPCP data to customized physical control plane (CPCP) data for the first managed forwarding element but not the second managed forwarding element; c) a third controller instance for receiving UPCP data generated by the first controller instance, identifying the second controller instance as the controller instance responsible for generating the CPCP data for the first managed forward element, and supplying the received UPCP data to the second controller instance.