This paper proposes an optimization method for the design of large scale confederation based BGP networks. We propose a graph based model and an associated metric to evaluate the reliability of large scale autonomous systems. We propose and validate an effective methodology to find the optimal design for a given physical topology. According to our experiments, we consider that replacing the traditional IBGP topology by an appropriate confederation design could increase at the same time the scalability and the reliability into the domain. Our work might be a step further towards a large scale confederation deployment.