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A two-level method of fast re-routing with load balancing in a software-defined network (SDN) is proposed. The novelty of the method consists, firstly, in the introduction of a two-level hierarchy of calculating the routing variables responsible for the formation of the primary and backup paths, and secondly, in ensuring a balanced load of the communication links of the network, which meets the requirements of the traffic engineering concept. The method provides implementation of link, node, path, and bandwidth protection schemes for fast re-routing in SDN. The separation in accordance with the interaction prediction principle along two hierarchical levels of the calculation functions of the primary (lower level) and backup (upper level) routes allowed to abandon the initial sufficiently large and nonlinear optimization problem by transiting to the iterative solution of linear optimization problems of half the dimension. The analysis of the proposed method confirmed its efficiency and effectiveness in terms of obtaining optimal solutions for ensuring balanced load of communication links and implementing the required network element protection schemes for fast re-routing in SDN.K e y w o r d s: SDN, flow-based model, fast re-routing, load balancing, link utilization, primary path, backup path, calculations hierarchy

In this paper, in order to increase the scalability and fault-tolerance of routing solutions the hierarchical method of interarea fast rerouting in communication networks was presented. The method is based on the decomposed representation of the flowbased routing model with the introduction of the area interaction conditions to ensure connectivity of the inter-area routes. The model includes conditions for border routers protection, adapted for both single path and multipath routing. During the research it was established that the efficiency of the proposed method in terms of the speed of the coordinating procedure convergence was most influenced by the number of border routers and the implemented routing strategy.