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Service Level Agreements (SLAs) are common means to define obligations between network/service providers and customers in business relationships. The terms that define the guaranteed availability for a given period are important elements of these contracts. The appropriate selection of values is difficult due to the large number of variables involved, the complexities of the network and service provision and the computational challenge posed by the transient solution required to analyze the system. In this work we use operational data to show the impact of dependent failures on the fulfillment of SLAs. We simulate the behavior of multiple network connections that use shared backup protection. We evaluate the SLA success probability using two different methods. First we apply trace driven simulation combined with random circular shifting. Second we develop a model that uses Monte Carlo techniques based on the same operational data. This approach includes the characterization of Up/Down times on each network component and the use of a model able to generate correlated samples based on the fitted marginal distributions. Finally we analyze the probability density function of the interval availability for different observation periods under independent and correlated failures.