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The path towards realizing peta-scale computing is increasingly dependent on scaling up to unprecedented numbers of processors. To prevent the interconnect architecture between processors from dominating the overall cost of such systems, there is a critical need for interconnect solutions that both provide performance to ulta-scale applications and have costs that scale linearly with system size. In this work we propose the Hybrid Flexibly Assignable Switch Topology (HFAST) infrastructure. The HFAST approach uses both passive (circuit switch) and active (packet switch) commodity switch components to deliver all of the flexibility and fault-tolerance of a fully-interconnected network (such as a fat-tree), while preserving the nearly linear cost scaling associated with traditional low-degree interconnect networks. To understand the applicability of this technology, we perform an in-depth study of communication requirements across a broad spectrum of important scientific applications, whose computational methods include: finite-difference, latticebolzmann, particle in cell, sparse linear algebra, particle mesh ewald, and FFT-based solvers. We use the IPM (Integrated Performance Monitoring) profiling layer to gather detailed messaging statistics with minimal impact to code performance. This profiling provides us sufficiently detailed communication topology and message volume data to evaluate these applications in the context of the proposed hybrid interconnect. Overall results show that HFAST is a promising approach for practically addressing the interconnect requirements of future peta-scale systems.