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We study SRLG (Shared Risk Link Group) failure monitoring and localization in all-optical WDM (Wavelength Division Multiplexing) networks. All links in each SRLG are logically grouped as a whole, and they fail at the same time when the SRLG failure event occurs. To achieve fast SRLG failure localization, monitoring is carried out at the optical layer using the recently proposed monitoring trail (m-trail) structure. By formulating an ILP (Integer Linear Program), we optimally solve the m-trail allocation problem to achieve unambiguous SRLG failure localization with the minimum monitoring cost. We claim that our work provides the first study in optimally allocating free-routed m-trails for achieving fast and unambiguous SRLG failure localization, with flexible tradeoff between the monitor cost and the bandwidth cost (i.e., supervisory wavelength-links).