lop; after that the packet is trans- ferred again at layer 2 to be sent to the output transmission system. In case MPLS is used in the router, besides the IP forwarding table (i.e., present if the router maintains also IP routing functionalities), there is a label forwarding table that is used to build the MPLS tunnels. The procedure to build the different table and to use them to route packets can be briefly described as follows. The router on the ground of the network knowledge and on the working of the selected routing protocols builds the routing table. If MPLS traffic engineering is used via a cen- tralized control, it forces some elements of the routing table to represent traffic engineered tunnels. The routing table is continuously updated based on the evolution of the network state from a traffic and resource availability point of view. For example, failures or addition of new equipment force a recalculation of the routing table. This is why automatic discovery is SO important to guarantee efficient routing. Once the routing table is calculated, depending on the need of performing either IP or MPLS operation, an IP forwarding table or an MPLS correspondence table is constructed. The IP forwarding table contains not only a correspondence between input and output ports SO that once the IP address of a packet is known also the destination output port can Telecommunication Networks be obtained, but also a set of information that are needed to forward correctly the packet into the layer 2 in the next hop. As a matter of fact, when a packet is forwarded by the routing processor and it arrives at the destination queue, before the transmission to next network hop it has to be enveloped again into the layer 2 envelop. The MPLS label correspondence table contains the correspondence between input and output labels that allows the packet to be directed toward the correct port. The difference between the IP forwarding table and the label correspondence table is that the first is tem- porary, there is no connection con