In this paper, we discuss the optimal design of telecommunications networks prone to failure. The problem concerns mid-range planning objectives. The purpose of the model is to give insight on the cost of protecting the network against failures by balancing the choice of a base routing of the demands and the investment in spare to protect the routing against elementary breakdowns. The problem in formulated as a large linear programming model. To exploit the structure we resort to a decomposition scheme with two nested levels. At the higher level, we propose two different schemes: a Lagrangian relaxation and a decomposition "à la Benders". The transformed problem is solved by a cutting plane method. We compare two such methods, the Cheney-Goldstein-Kelly method and the analytic center cutting plane method. The methods are tested on a set of realistic problems