# Router Activation Heuristics for Energy-Saving ECMP and Valiant Routing in Data Center Networks
## 1 Introduction  
Increasing demand for advanced computing services and transmission quality requires expansion of the existing network and server architecture [1; 2; 3]. The increase in the number of computing units and network devices results in a significant increase in power consumption while at the same time increasing the number of devices that are not constantly in use. Redundant nodes in the network are necessary to ensure the scalability of services, to enable the handling of an increased number of customers in the future, and, above all, due to changes in demand for computing power and network capacity that are difficult to predict. Data center (DC) energy efficiency is often not critical in designing a computer network architecture for data exchange between computing servers. However, it is evident that to support computationally intensive services, new-generation networks able to perform complex operations in a scalable way and ensure that the expected quality of service is provided. Therefore, network devices are expected to have increased computing power and bandwidth, resulting in growing energy requirements. The higher network maintenance costs increase the costs incurred by the end customer. In general, ignoring the energy cost when designing a data center can result in an overdesigned infrastructure with an increased maintenance cost and it is not even required to provide adequate safety margins in terms of available computing power and bandwidth of existing links. Therefore, energy awareness is an essential aspect of modern data center design and management, and the optimizationof total power consumption in data center networks (DCNs) is a significant research issue. The main challenge is developing novel technologies that reduce energy consumption in such infrastructures. New energy-efficient routing algorithms, methods, and control frameworks for dynamic power management in energy-aware computer networks have been designed and developed to achieve the desired trade-off between the data center performance according to the DCN capacity, current requirements, and power consumption [4; 5; 6; 7; 8; 9]. The aim is to reduce the gap between the capacity provided by a data center network for data transfer and the current demand for data exchange between computing nodes and increase network resilience to unpredictable spikes in traffic load. In particular, the energy consumption in a network can be minimized by switching off or idlingenergy-consuming components such as switches, routers, line cards, and communication interfaces and by reducing the speed of processors and link speed [10; 11; 12; 13]. The data transfer aggregation along as few devices as possible instead of balancing traffic in a whole computer network can be successfully used for energy-efficient dynamic management in local area networks (LANs) and wide area networks (WANs). Unfortunately, in the case of heavily used DCNs, where the critical task is to guarantee fast and reliable data transfer between computing servers, it may not be possible to disable multiple network devices. In this case, it is necessary to determine the path online with sufficient bandwidth, assuming the lowest possible power consumption. In this paper, we overview the network topologies and routing algorithms used in real data centers. We assume that the data center can offer different services and that the data center network must carry the associated transmissions. We consider a software-defined network (SDN) with a central controller when the control plane is separated from network nodes. Thus, the routing may be calculated in a centralized manner. To ensure high availability, the redundant central controller may be implemented. The primary attention we pay to energy-efficient routing: We formulate the energy-saving DCN management optimization problem. We discuss the limitations of formal analysis and numerical optimization in large-scale DCN management.