Application-Layer Multicast has become a promising class of protocols since IP Multicast has not found wide area deployment in the Internet. Developing such protocols requires in-depth analysis of their properties even with large numbers of participants---a characteristic which is at best hard to achieve in real network experiments. Several well-known simulation frameworks have been developed and used in recent years, but none has proved to be fitting the requirements for analyzing large-scale application-layer networks. In this paper we propose the OverSim framework as a promising simulation environment for scalabe Application-Layer Multicast research. We show that OverSim is able to manage even overlays with several thousand participants in short time while consuming comparably little memory. We compare the framework's runtime properties with the two exemplary Application-Layer Mutlicast protocols Scribe and NICE. The results show that both simulation time and memory consumption grow linearly with the number of nodes in highly feasible dimensions.