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Introducing SDN into an existing network causes both deployment and operational issues. A systematic incremental deployment methodology as well as a hybrid operation model is needed. We present such a system for incremental deployment of hybrid SDN networks consisting of both legacy forwarding devices (i.e., traditional IP routers) and programmable SDN switches. We design the system on a production SDN controller to answer the following questions: which legacy devices to upgrade to SDN, and how legacy and SDN devices can interoperate in a hybrid environment to satisfy a variety of traffic engineering (TE) goals such as load balancing and fast failure recovery. Evaluation on real ISP and enterprise topologies shows that with only 20% devices upgraded to SDN, our system reduces the maximum link usage by an average of 32% compared with pure-legacy networks (shortest path routing), while only requiring an average of 41% of flow table capacity compared with pure-SDN networks.
Hong et al. (Mon,) studied this question.