  With the dynamic demands and stringent requirements of various applications,
networks need to be high-performance, scalable, and adaptive to changes.
Researchers and industries view network softwarization as the best enabler for
the evolution of networking to tackle current and prospective challenges.
Network softwarization must provide programmability and flexibility to network
infrastructures and allow agile management, along with higher control for
operators. While satisfying the demands and requirements of network services,
energy cannot be overlooked, considering the effects on the sustainability of
the environment and business. This paper discusses energy efficiency in modern
and future networks with three network softwarization technologies: SDN, NFV,
and NS, introduced in an energy-oriented context. With that framework in mind,
we review the literature based on network scenarios, control/MANO layers, and
energy-efficiency strategies. Following that, we compare the references
regarding approach, evaluation method, criterion, and metric attributes to
demonstrate the state-of-the-art. Last, we analyze the classified literature,
summarize lessons learned, and present ten essential concerns to open
discussions about future research opportunities on energy-efficient softwarized
networks.
