ices VOIP services Cloud services Mobility infrastructure and internet access MPLS technology [11] is used to implement the packet core. Figure 13.10 uses the standard industry framework in which edge functions are referred to as the provider edge (PE) and core functions as the provider (P); the PE's encapsulate packets into MPLS overhead for transport across the P core. MPLS is used to build a set of tunnels that deliver traffic from an ingress P router across the core to the edge-facing port on an egress P router. Each P router advertises to its subtending PEs the tunnels the PE should use to get information to distant egress PEs. The ingress PE packages its service-specific data into an MPLS container and pushes it into the tunnel advertised for the Network Core Packet/Optical Core Customer access Ingress PE Egress PE FIGURE 13.10 MPLS packet core. egress PE. The core delivers the MPLS packet to the egress PE choosing the best route available through the core. It does SO without needing to know anything about the contents of the container or the eventual end customer the contents will be sent to. Its job is simply to efficiently and reliably transport the MPLS container to the egress PE. The ingress PE only needs to specify which egress PE the container should be delivered to; it does not need to worry itself with the messy details of how to best get it there. The next sections will discuss how the tunnels are routed and built, their resiliency, and their adaptability. For simplicity, we have only shown the tunnels originating from Pa in Figure 13.10. Tunnels do originate from all other P routers as well. The nodes in the network use an Interior Gateway Protocol (IGP)* to exchange routing informa- tion including topology (which nodes have adjacency to others), link metric, and state (link up/link down) information that enable each node to make informed decisions regarding the best path to route tunnels toward a destination. Typically, mileage is used as the metric causing nodes to route along the lowest latenc