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The design and engineering of networks requires the consideration of many possible configurations (different network topologies, bandwidths, traffic and policies). Network engineers may use network simulation to evaluate changes in network configuration; but detailed, packet-level simulation of many alternatives would be extremely time consuming. This paper introduces the concept of scenario pre-filtering - rather than perform detailed simulation of each scenario, we propose to quickly evaluate (pre-filter) all scenarios in order to select only the relevant scenarios, and discard those that are clearly too over- or under-provisioned. To rapidly evaluate scenarios, we have developed several new analytical techniques to quickly determine the steady-state behavior of the network with both bulk and short term TCP flows. These techniques apply to arbitrary topologies and routers that use both drop-tail and RED queuing policies. Since we are only interested in selecting the interesting scenarios for detailed simulation, the answers need only be approximate. However we show that the accuracy is typically within 10% of detailed simulation. More importantly, these techniques are 10-300× faster than detailed simulation, and, hence, pre-filtering is a promising technique to reduce the total simulation time when many scenarios must be considered.