Method for placing bypass capable nodes in two-layer networks

In two-layer networks there are various ways to improve virtual and physical topology and capacity allocations in terms of cost. One way is introducing bypass links, to reduce the amount of switching in the upper layer. Our approach for finding the bypass links and the corresponding bypass nodes is based on a mixed integer programming approach. Our starting point is a network without bypass functionality. This is basically a one-layer network, as all traffic is switched in the upper layer. In a first step we find the optimum capacity allocations in this network, taking the interface granularities into account. After this we calculate the resulting transit traffic in all nodes. If the transit traffic between two next nearest neighbour nodes is larger than a certain threshold, a bypass link between them is introduced between them. The new virtual (upper layer) topology is then used to optimize capacity allocations again. This procedure leads to considerable cost reductions, but is computationally much simpler than a full two layer optimisation including all possible bypass links.