t to the incorporation into the router several functionalities that are not typical of a datagram-based network. The main evolution from the point of view of the architecture of the router itself is the introduction into the equipment of MPLS first and then GMPLS. MPLS and GMPLS switching are more similar to layer 2 functionalities, SO configuring an IP/LSR as a machine having switching capabilities at different layers. The separation, however, is not SO sharp as it would be in a potential machine incorpo- rating a layer 2 Ethernet switch and on top of it a layer 3 IP router. An example is consti- tuted by QoS, that are positioned frequently in both layer 2 and layer 3, somehow making exceptions to the rule of layer functionalities separations. Telecommunication Networks This situation is historically a consequence of the quite informal environment that gave rise to Internet whose aggregation community is the IETF. The fact is, however, that this approach had a SO huge success that it is now the reference point for the new network architecture (see ASON/GMPLS architecture in Chapter 8). 7.8.1 Generalities on IP/MPLS Routers and Routers Classification The process that an IP packet undergoes while traversing a router and being directed by it into the correct propagation direction is shown schematically in Figure 7.52 and it can be synthetized as follows [1]: The IP packet arrives at the adaptation layer between layer 2 and layer 3 and the layer 2 header is terminated. The IP checksum is calculated to verify packet integrity; if the checksum is not correct the packet is canceled. The packet is stored into the input queue of the input interface card. An interrupt is sent to the routing processor to inform it that a packet is waiting to be processed. When the routing processor answers the interrupt, it receives the IP header from the interface card where the packet is stored. Routing Forwarding table table Forwarding Drop the address Offline Routing packet update processor LC up LC up Routing processor Layer 3 Layer 2