can have millions of constraints and variables. This easily makes the existing centralized TE intractable because of its exponential worst-case complexity [100]. Adding more compute resources to the TE application does not fun- damentally address the scalability challenge given the rapid growth of the network size. Having multiple SDN controllers (e.g., ONOS [81]) does not by itself reduce the TE complexity because the flow opti- mization finally is assigned to a single instance, needing to process the entire TE states (e.g., topology, traffic matrices, constraints). To address these issues, we present a software-defined TE solution, called SdnTE, that can scale to manage traffic in very ISP networks. SdnTE enables ISPs to execute TE rapidly and frequently using a logi- cally centralized software in their network. At its heart, SdnTE sacri- fices a small amount of network throughput compared to the global, optimal TE techniques to substantially accelerate the TE computation by several orders of magnitude (e.g., from hours to seconds). SdnTE enables ISPs to quickly respond to unexpected changes in traffic pat- terns (e.g., unpredictable rush hours), and thus highly utilize their net- work resources. In more detail, SdnTE builds up a hierarchical control plane for ISPs and designs a novel recursive, distributed TE applica- tion on it. For TE scalability, SdnTE systematically partitions the global TE task throughout hierarchy by creating a logical TE region for each control node in the hierarchy. Each node is responsible for optimizing flows in its region using linear optimization, while coordinating with its parent and children indirectly. Historically, the recursive topology aggregation and hierarchy tech- niques have been used to minimize the routing table size in routing protocols (e.g., PNNI [101], Nimrod [102]) in different network technologies. These designs are limited to the distributed link-state or source-based routing protocols and cannot be leveraged for global flow optimization. Hierarchical SDN contro