Abstract: HPC system procurement with a fixed budget is an optimization problem with many trade-offs. In particular, the choice of an interconnection network for a system is a major choice, since communication performance is important to overall application performance and the network makes up a substantial fraction of a supercomputer’s overall price. It is necessary to understand how sensitive representative jobs are to various aspects of network performance to procure the right network. Unlike previous studies, which used mostly communication-only motifs or simulation, this work employs a real system and measures the performance of representative applications under controlled environments. We vary background congestion, job mapping, and job placement with different levels of network tapering on a fat tree. Overall, we find that a 2:1 tapered fat tree provides sufficiently robust communication performance for a representative mix of applications while generating meaningful cost savings relative to a full bisection bandwidth fat tree. Furthermore, our results advise against further tapering, as the resulting performance degradation would exceed cost savings. However, application-specific mappings and topology-aware schedulers may reduce global bandwidth needs, providing room for additional network tapering.