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Today, the complexity of ISPs' networks make it difficult to investigate the implications of internal or external changes on the distribution of traffic across their network. In this article we explain the complexity of building models of large ISPs' networks. We describe the various aspects important to understanding the routing inside an AS. We present an open source routing solver, C-BGP, that eases the investigation of changes in the routing or topology of large networks. We illustrate how to build a model of an ISP on a real transit network and apply the model on two "what-if" scenarios. The first scenario studies the impact of chances in the Internet connectivity of a transit network. The second investigates the impact of failures in its internal topology.