Oak Ridge National Laboratory's Summit supercomputer and Lawrence Livermore National Laboratory's Sierra supercomputer utilize InfiniBand interconnect in a Fat-tree network topology, interconnecting all compute nodes, storage nodes, administration, and management nodes into one linearly scalable network. These networks are based on Mellanox 100-Gb/s EDR InfiniBand ConnectX-5 adapters and Switch-IB2 switches, with compute-rack packaging and cooling contributions from IBM. These devices support in-network computing acceleration engines such as Mellanox Scalable Hierarchical Aggregation and Reduction Protocol, graphics processor unit (GPU) Direct RDMA, advanced adaptive routing, Quality of Service, and other network and application acceleration. The overall IBM Spectrum Message Passing Interface (MPI) messaging software stack implements Open MPI, and was a collaboration between IBM, Mellanox, and NVIDIA to optimize direct communication between endpoints, whether compute nodes (with IBM POWER CPUs, NVIDIA GPUs, and flash memory devices), or POWER-hosted storage nodes. The Fat-tree network can isolate traffic among the compute partitions and to/from the storage subsystem, providing more predictable application performance. In addition, the high level of redundancy of this network and its reconfiguration capability ensures reliable high performance even after network component failures. This article details the hardware and software architecture and performance of the networks and describes a number of the high-performance computing (HPC) enhancements engineered into this generation of InfiniBand.