e equal to 2 due to the presence of the backbone and to the way in which the areas have been constructed. The routing between nodes in different areas is performed in two steps: first the route is individuated in the area topology, then, knowing for each traversed areas the incoming and the outcoming nodes, the path route is specified also within areas. 8.3.2.2 IS-IS Routing Protocol IS-IS is designed and optimized to provide routing within a single network domain, while passage through a domain interface for multidomain GLSPs have to be realized with a different protocol (e.g., the BGP, see Chapter 3, Table 3.1 IP protocol suite). IS-IS is based on the concept of routing domains. An IS-IS routing domain is a network in which all the equipments that perform GLSP routing run the integrated IS-IS routing protocol to sup- port intradomain exchange of routing information. Convergent Network Management and Control Plane Area 2 Area 2 Area 1 virtual Area 1 virtual Backbone (Area 0) Backbone (Area 0) Area 3 Area 3 virtual Network topology with GLSRs Area topology with virtual nodes FIGURE 8.13 GMPLS partition of the network in routing areas and backbone. On the right of the figure the virtual topology where each routing area is represented by a virtual node is represented. The underlying goal is to achieve consistent routing information within a domain by having identical link-state databases on all routing entities in that area. Hence, all routing entities in an area are required to be configured in the same way. IS-IS protocol supports a two-level hierarchy for managing and scaling routing in large networks. A network domain can be divided into small segments known as areas. The topology resulting from the substitution of each area with a virtual node and each con- nection between nodes in different areas with a connection between the corresponding virtual nodes is called area topology. This construction allows hierarchical routing to be carried out within the domain. This means that the routing between nodes in diffe