This paper presents SHEAR, a highly available hybrid network architecture which marries distributed legacy protocols with Software-Defined Networking (SDN) technology. SHEAR is based on a small deployment of Openflow switches which serve as “observability points”: SHEAR leverages legacy distributed control plane protocols to detect and localize failures, but outsources the actual failover logic to the logically centralized SHEAR controller, which can make faster and more informed routing decisions. Moreover, the Openflow switches are used to logically decompose the legacy network into loopfree components, enabling a simple and flexible traffic-engineering. The deployment problem solved by SHEAR can be seen as a new variant of a network tomography problem, and may be of independent interest. Our simulations show that in enterprise networks, between 2 to 10 % Openflow switches are sufficient to implement SHEAR. We also report on our prototype implementation which detects a failure and reroutes traffic in less than .3 seconds in our testbed—much faster than what is achieved by the less flexible and distributed legacy protocols. More generally, SHEAR demonstrates that in contrast to common belief, operating a hybrid softwaredefined network can be simple, and given its benefits, a partial Openflow deployment may even be a long-term solution.

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