y path but there can be other considerations. Note, for example, that the path of the tunnel from P to P was via Pb. There is a path via that was not chosen because it has a higher cumulative cost (sum of metrics of the links in the path). 13.2.2 EVOLUTION OF MPLS Figure 13.11 shows the major evolutionary steps of the MPLS-based packet core. 13.2.2.1 Basic MPLS Transport The first phase of the MPLS journey focused on building a single MPLS-based core to provide com- mon transport for all services. This was the first realization of the edge/core paradigm and produced a service-agnostic core. In building the MPLS core, tunnels need to be created and routed through the core. The technical term for a tunnel is a label switched path (LSP). Edge routers insert a small amount of additional overhead to each packet (beyond the IP overhead) to enable MPLS forwarding across the core. The overhead contains a label, which is the identifier for the tunnel, that is used to indicate the desired The two most popular routing protocols are Open Shortest Path First (OSPF) and IS-IS. There are many resources read- ily available for the interested reader. Building the Network of the Future Distributed Traffic Transport Engineering Centralized Fast Re-Route Hitless Re-arrangements Bandwidth Aware Routing FIGURE 13.11 Evolution of MPLS packet core. egress PE. Each P router has a set of subordinate PEs. It learns about their existence through the routing protocol mentioned earlier. Each P router advertises to each of its neighbors the label they should use to reach each PE. The P routers exchange this information with their neighbors, both their subordinate PEs as well as other P routers. Each node is free to assign a new label for a destina- tion PE; there is no requirement that it reuse the labels that it received from its adjacency. But a given router must advertise the same label for each PE on all of its interfaces and the label assigned for each PE must be locally unique. The protocol used for label distribution is called Label Dis