Interdomain routing is a massive distributed computing task that propagates topological information for global reachability. Today's interdomain routing protocol, BGP4, is exceedingly complex because the wide variety of goals that it must meet - including fast convergence, failure resilience, scalability, policy expression, and global reachability - are accomplished by mechanisms that have complicated interactions and unintended side effects. The complexity of wide-area routing configuration and protocol dynamics requires mechanisms for expressing wide-area routing that adhere to a set of logical rules. We propose a set of rules, called the routing logic, which can be used to determine whether a routing protocol satisfies various properties. We demonstrate how this logic can aid in analyzing the behavior of BGP4 under various configurations. We also speculate on how the logic can be used to analyze existing configuration in real-world networks, synthesize network-wide router configuration from a high-level policy language, and assist protocol designers in reasoning about new routing protocols.