1. A network control system for generating physical control plane data for managing a set of managed forwarding elements that implement forwarding operations associated with a logical datapath set, the system comprising: a first controller computer having one or more processing units for executing a program for (i) receiving logical control plane data that define the logical datapath set, and (ii) converting the logical control plane data to universal physical control plane (UPCP) data that define forwarding behaviors of the set of managed forwarding elements in terms of generic expressions of forwarding attributes of every managed forwarding element in the set of managed forwarding elements; and a second controller computer having one or more processing units for executing a program for (i) receiving the UPCP data from the first controller computer, and (ii) converting the UPCP data to customized physical control plane (CPCP) data for defining a forwarding behavior of a particular managed forwarding element in terms of expressions of forwarding attributes that are specific to the particular managed forwarding element and not to the remaining managed forwarding elements in the set.