f ATM that allows trunks of circuits to be transported through a core part of the network; outside of this core part, VPI switching might not be used, and the VCI identifiers can be swapped. In each switch a table is maintained that maps an incoming label to an outgoing port and new label value. This table provides a simple relationship between incoming and outgoing cells. This can be contrasted with the more complex routing table lookup required by IP routers, where the destination IP address of an incoming packet has to be matched against the longest prefix in the routing table, which can contain more than 150,000 entries for a core Internet router. Having introduced the label-switching table (VPI/VCI table in ATM), we should note that there needs to be a mechanism to set the values in the table. We will see later that MPLS can use a variety of protocols to support this. ATM has its own set of protocols: a User Network Interface (UNI) between an end device and the first switch and a Private/Public ATM Cell Payload VCI = 2 VPI 3 Fig. 13.1 Label swapping as used by ATM. Introduction Network-Network Interface (PNNI) to coordinate path configuration through an ATM network. The Evolution of MPLS MPLS is the result of a merging of a number of competing technologies that were put in place to solve the problems of transporting IP over circuit-switched networks, which were widely used in the 1990s. At that time ATM was widely deployed, and some believed that it would be a candidate for an end-to-end communication technology. However, it quickly became clear that IP was to become the dominant end protocol because of its widespread deployment in operating systems. Additionally, Ethernet was substantially cheaper to deploy in the Local Area Network (LAN) than ATM. Consequently, ATM was relegated mostly to a carrier technology for providing overlay networks to support connectivity between IP networks. An overlay provides a virtually transparent pipe to a higher-layer protocol such that the two do not interact except for enc