ne is based on the SO called Dijkstra algorithm (from the name of the inventor), which is an algorithm that is able to find in a graph with tagged links the shortest path between two given nodes. In GMPLS, what should be the tagging of the links of the network is not prescribed in a mandatory way; generally every link is associated to its length and the GLSP are routed along the shortest path in term of kilometers. This is a rea- sonable solution since the cost of a DWDM system is roughly proportional to its length due to the presence of EDFA sites. Thus, since if I occupy preferably the shortest links I will have to upgrade them sooner, this strategy tends to minimize the cost of network upgrades. However, in specific situations, it is possible to tag the network links with other num- bers. For example, a competitive carrier leasing fibers from another company could tag the link with the leasing price SO to use always the cheaper links. This direct routing through the topology of the network with the Dijkstra algorithm becomes increasingly complex while the network dimension increases, SO that the routing process has to be simplified in some way if OSPF-TE has to scale up to the dimension of an incumbent carrier network. The solution to this problem is the division of the network in routing areas and the execu- tion of the routing in a hierarchical manner. In particular, OSPF-TE individuates in the network a set of routing entities (GLSR) that are suitably distributed to constitute the so-called network backbone. All the other GLSRs are divided in routing areas constituted by nearby nodes. Each distinct routing area has to be in contact with the backbone at least in two points to assure effective network survivability. An example of this hierarchical organization of the network is reported in Figure 8.13. Staring from this structure, a new higher level topology (the area topology) is con- structed substituting all the routing areas with virtual nodes; this new topology has for sure at least interconnection degre