cept in IP. Thus, changing the network condition, the change in the routing table can provoke changes in the IP forwarding table and packets directed toward the same destination (also packets belonging to the same TCP connection) are routed toward different output ports. On the contrary, the MPLS label correspondence is a permanent thing, once a correspon- dence has been set up it cannot be canceled simply because the situation of the network is changed and the routing table is changed, but a complete process of teardown is needed to cancel a label correspondence. Thus, MPLS tunnels behave like virtual connections and packets having at the LSR input the same MPLS label are routed toward the same output port whatever the situation of the network is, up to the moment in which the MPLS tunnel is tear down. An exception is of course the failure of the output link or of the output interface when MPLS restoration mechanisms automatically set up an alternative tunnel via suitable protocols. Naturally, in case of MPLS routers MPLS label correspondences have to be distributed along the network SO that a tunnel can be recognized by the various LSRs. As explained in Chapters 3 and 8 it is done via suitable labels distribution protocols. Originally MPLS was designed to simplify the router's operation: as a matter of fact, it relies on the idea that all the datagrams belonging to the same connection and thus directed to the same destination are tagged with a label and are switched on the same route. Thus, the orgin of MPLS relies on the idea that the labels correspondence table is simpler than the IP forwarding table. This original intention, however, is no more the most important reason pushing the adoption of MPLS especially in backbone IP networks. Even because frequently an LSR setup a SO high number of tunnels that the label correspondence table is almost SO complex as the IP forwarding table. The presence of fixed tunnels conveying datagrams belonging to the same com- munication is instead precious to perform network mo