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Spectrum elastic optical networks support flexible central frequency and spectrum assignment for lightpaths. When provisioning a new connection in an elastic optical network that allows traffic grooming, the control plane has to solve two problems: the electrical-layer routing and optical-layer routing and spectrum assignment (RSA). The electrical-layer routing determines how to route the new connection through a combination of new and existing lightpaths, while the optical-layer RSA decides how to establish new lightpaths under the spectrum-continuity constraint. The flexibility (e.g., bandwidth variability of lightpaths) provided by elastic optical networks makes it suitable for accommodating dynamic traffic. It is important and challenging to exploit the full potential of the flexibility when dealing with the above two problems. In this study, we propose a multi-layer auxiliary graph to jointly solve the electrical-layer routing and optical-layer RSA. Various traffic-grooming policies (objectives) can be achieved by properly adjusting the edge weights in the auxiliary graph. Also, we propose a spectrum reservation scheme that can efficiently utilize the bandwidth variability of lightpaths by reserving bandwidth for non-fully utilized lightpaths and grooming future connections onto them. We show that there is a tradeoff among different traffic-grooming policies, and the spectrum reservation scheme can be easily incorporated into various traffic-grooming policies and lead to a significant reduction in operational expenditure (OPEX) and better spectrum efficiency.