  We consider the problem of single link failure in an elastic optical network,
(also known as flex-grid WDM network). The task is to reroute optical
connections that go through the broken link using free capacity of other links
of the network. Nowadays, dynamic restoration gains popularity, in which the
possiblity of rerouting is only inspected after a link failure is detected.
Since the problem of recovery is NP-hard, heuristic algorithms are used to
either find such routes, or suggest that the routes do not exist. In order to
understand the quality of these heuristics, often mixed integer linear
programming is used to obtain exact positive and negative answers. We present a
detailed such model that checks whether restoration is possible without the use
of additional regenerators. This means, that the new light paths need to
satisfy a length constraint. As preprossing we apply a trimming procedure that
takes advantage of this length constraint, and significantly speeds up the
evaluation of these models. Our model is more general, and besides solving the
problem of link restoration, also solves the full problem of wavelength and
spectrum assignment.
