We develop Propane/AT, a system to synthesize provably-correct BGP (border gateway protocol) configurations for large, evolving networks from high-level specifications of topology, routing policy, and fault-tolerance requirements. Propane/AT is based on new abstractions for capturing parameterized network topologies and their evolution, and algorithms to analyze the impact of topology and routing policy on fault tolerance. Our algorithms operate entirely on abstract topologies. We prove that the properties established by our analyses hold for every concrete instantiation of the given abstract topology. Propane/AT also guarantees that only incremental changes to existing device configurations are required when the network evolves to add or remove devices and links. Our experiments with real-world topologies and policies show that our abstractions and algorithms are effective, and that, for large networks, Propane/AT synthesizes configurations two orders of magnitude faster than systems that operate on concrete topologies.