Davide Cherubini - PhD Thesis - UniCA Eprints

6.4 LP modelsThe second layer introduces the survivability constraints, which are expressedin different ways, depending on the used restoration technique. Furthermore,these constraints are presented as capacity constraints in order to guarantee a100% survivability level avoiding possible congestion scenarios.For each scenario, the occupation level of all links is evaluated and the maximumis considered as objective function to be minimized. This situation representsan upper bound of the global optimization process and a better objectivefunction can be defined.As shown in chapter 3, an optimal or at least sub-optimal configuration of theIS-IS metrics is the underlying condition for all the models. Such a set can befound using metaheuristic approaches presented in literature [43], [44],and [63].6.4.1 Model 1The basic idea of the first model is to find an optimal set of complementaryexplicit LSP tunnels, to be used in combination with conventional IS-IS routingprotocol, in order to reduce the occupation level of the network under single linkfailure condition.The link restoration technique is used and the backup paths are realized usingimplicit LSPs. It is advisable to recall that, for an explicit LSP the used linkshave to be specified by the network operator, while an implicit LSP automaticallydetermines its path using the IS-IS metrics.In terms of network management, this model specifies the following steps:1. Setup of the (sub)optimal IS-IS routing metrics previously calculated.2. Setup of the explicit LSPs determined by the model.3. No intervention of the network operators is required to setup the restorationconfiguration.The model is presented with the arc-path formulation as well as with the node-arcformulation.52