Parallel computers today are designed with larger number of processors than ever before, connected by large scale Interconnection Networks. Communication is the key to achieving high performance on such machines, making the study of Interconnection Networks important. Parallel simulations of Interconnection Networks present a unique problem characterized by fine-grained computation and strong dependence among events. The absence of large lookaheads makes it unsuitable to use a conservative simulation. Using an optimistic Parallel Discrete Event Simulation allows us to extract reasonable parallelism from this simulation. In this paper we present BigNetSim, an Interconnection Network simulator. We analyze its performance and present techniques related to enhancing performance and scaling it to a large number of processors on different artificial traffic patterns and real application logs. Inspite of the overheads of a parallel optimistic simulation, we have achieved a breakeven with sequential simulation at four processors and demonstrate perfect scaling to 128 processors.