ms or OXCs and LSRs with a very low number of wavelengths or interfaces, only one in the limit case. In this case, it can be beneficial to create a postprocessing routine that tries to eliminate these systems by rerouting the wavelengths on already existing equipment and verifies if the cost of the network can be lowered in this way. We do not detail how such post- processing can be implemented, technical literature on such heuristics is very rich, and the interested reader can start to approach the problem from [15]. To obtain the results presented in this chapter, no postprocessing is used since looking at the obtained solu- tions shows that the platform approximation is never SO important to change the results significantly. Considering the constraints, we have to maintain them linear if we want to formulate an ILP problem. The first set of conditions is the flow continuity across the nodes at every layer. This means that at every layer the traffic entering the node from client and line ports Convergent Network Management and Control Plane has to balance the traffic exiting from client and line port. Looking at the LSR, the continu- ity, coinciding with the bidirectional nature of the LSR, can be written as ((i,,k1,e1,]) (8.17) (8.18) It is important to note that even if (8.17) and (8.18) are constraints, they can be imposed directly on the routing matrix with the effect of reducing the independent variables and decreasing the overall complexity. This is particularly easy if the sparse representation is used for the routing matrix, where LSPs appear explicitly. In this case, only the variables representing an LSP in one direction have to be considered, SO halving number of elements of the pointers matrix. While looking at the OXC we have two continuities, one in terms of wavelengths and one in terms of LSPs. In terms of wavelengths we can write (8.19) which means that the wavelength matrix is symmetric. In terms of LSPs, the bidirectional condition is written as 1,M]) (8.20) (8.21) Last but not least the conti