One of the primary challenges facing scalable network emulation and simulation is the overhead of storing network-wide routing tables or computing appropriate routes on a per-packet basis. We present an approach to routing table calculation and storage based on spanning tree construction that provides an order of magnitude reduction in routing table size for Internet-like topologies. In our approach, we maintain a variable number of spanning trees for a given topology and choose the path between two hosts in each tree, choosing the shortest. We also populate offline a negative cache of actual shortest paths for source-destination pairs — typically a few percent of the total — where the lookups result in sub-optimal routes. We have implemented our technique in a popular network emulator, ModelNet, and show that our enhanced version can emulate Internet topologies 10-100 times larger than previously possible.