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In IP-over-WDM networks the logical topology consists of a set of lightpaths that are routed on top of the physical fiber topology. Hence a single fiber cut can lead to multiple logical link failures. We study the impact of lightpath routing on network survivability and spare capacity requirements. We show that poor routings can lead to significant increase in spare capacity requirements and develop new metrics for assessing the survivability of different lightpath routings. Finally, we use these metrics to develop joint lightpath routing and capacity assignment algorithms that significantly reduce the spare capacity requirements of IP-over-WDM networks.