ituations are represented. Physical fiber connections are routed through multifiber cables and links having different destinations are built over a cable structure that generally has been deployed without any knowledge of the overlay network. Thus, also links that have completely different source and destinations can share in part of their path the same fiber cable. If the cable is cut, for example, due to civil works, all those links goes down, thus they have to be put together in the same SRLG. The definition of SRLGs is fundamental when defining protection or restoration strate- gies. As a matter of fact, when the protection capacity is preprovisioned, it is fundamental to avoid that a link is protected by another link in the same SRLG, otherwise a single fail- ure could hit both working and protection link, making the protection useless. In the case of restoration, when routing of the restoration path is performed, also links in the same SRLG has to be avoided. This is why the SRLG is indicated among the information related to the routing protocol. Connection topology Cable topology Shared FIGURE 8.12 Concept of SRLG. Telecommunication Networks The second field that is worth a comment is the interface switching capable descrip- tion. It is clear that a link will terminate on the same type of interfaces and this field let the routing protocol know what kind of interfaces there are at the link extremes. During GLSPs, this information can be very useful to balance sparing of higher layer interface use achieved by transparent pass-through of the traffic not destined to the node with transmis- sion resources sparing achieved by grooming (see Section 8.4: Design and Optimization of ASON/GMPLS Networks). Once the OSPF-TE has flooded the network and created all the nodes capable of routing GLSPs, a database of the routing area topology, the OSPF-TE routing engine can start every time there is a GLSP to route either if it is a new path or it is needed to carry out restoration of a failed path. The OSPF-TE routing engi