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Spare capacity allocation serves as one of the most critical tasks in dynamic GMPLS networks to meet the stringent network availability constraint stipulated in the SLA of each connection. In this paper, an availability-aware spare capacity reconfiguration scheme based on shared backup path protection (SBPP) is proposed, aiming to guarantee the E2E availability of each LSP. We first provide an E2E availability model for a SBPP connection that is composed of a working and a SRG-disjoint shared backup LSP pair in the presence of all possible single, and dual simultaneous failures. Partial restoration is identified to further improve the capacity efficiency, and achieve finer service differentiation. For this purpose, restoration attempt is defined as a parameter for each connection that can be manipulated at the source node when the spare capacity of each link is scheduled. Based on the developed model, a linear program (LP) is formulated to perform inter-arrival spare capacity reconfiguration along each pre-determined shared backup LSP to meet the availability constraint of each connection. Simulation is conducted to verify the derived formulation, and to demonstrate the benefits gained in terms of the spare capacity saving ratio, where the conventional SBPP scheme that achieves 100% restorability for any single failure is taken as a benchmark. We will show that the simulation results validate the proposed E2E availability model, where a significant reduction on the required redundancy can be achieved in the effort of meeting a specific availability constraint for each SBPP connection.