Nowadays, although the performance of network devices has improved a lot, they consume more and more energy. Therefore, network energy efficiency is increasingly important in this era. In this paper, a software approach to energy-efficient networks is proposed and an application, which can make mid-scale networks energy efficient is demonstrated. In the proposed approach, some idle network interfaces of routers are disabled when the network load is low. Traffic flow converges on certain network paths, as disabled interfaces make some paths unavailable. When network load gets high, disabled interfaces can be re-enabled to ease the traffic pressure and network traffic disperses to different paths. A dynamic routing protocol is configured in order to keep connectivity when the topology changes. This approach is realized by an application, which is developed using Juniper Junos Software Development Kit (Junos SDK) and operates on Juniper programmable routers. Multiple entities of the application operating on different routers in the network form a distributed system. With regards to functionality, the application senses network traffic load; communicates among entities on other routers; determines and executes interface disabling and enabling. A virtual network built by Juniper virtual network simulator is used to test this application. Results reveal that a reasonable amount of energy can be saved by disabling idle interfaces while maintaining the connectivity.