one 13.2.2 Evolution of MPLS 13.2.2.1 Basic MPLS Transport 13.2.2.2 Fast Reroute and Hitless Rearrangements 13.2.2.3 Distributed Traffic Engineering: Bandwidth Aware Routing 13.2.3 Segment Routing 13.2.3.1 Packet Forwarding under Normal Conditions 13.2.3.2 Explicit Routing 13.2.3.3 Faster Restoration Using Segment Routing 13.2.3.4 Segment Routing Benefits 13.2.3.5 Segment Routing versus RSVP-TE 13.2.4 Core Router Technology Evolution 13.2.5 Route Reflection 13.3 SDN Control of the Packet/Optical Core 13.3.1 Centralized TE 13.3.2 Multilayer Control 13.3.3 Optimization Algorithm Implementation Acknowledgments References The network core is the layer of the network that provides high-speed connectivity between the edge platforms described in Chapter 12. The data plane of the network core comprises an optical layer and an Multi-Protocol Label Switching (MPLS)-based packet layer. Historically, networks were developed and deployed for specific applications (e.g., Internet ser- vices, private VPN services, mobility services, etc.). As the industry evolved to all-IP networks, service providers were motivated by economic and operational considerations to migrate their application-specific silo networks to a model that consisted of application-specific edges intercon- nected by a common, converged core (Figure 13.1). The current focus on NFV/SDN is causing further evolution of the network core to include a software-controlled, reconfigurable open optical transport layer, the introduction of high-density/ low-cost merchant silicon-based packet technologies, the virtualization of the route reflector (RR) function, and a shift from distributed protocols to a hybrid control architecture that marries the best of distributed and centralized SDN control. The following sections will expand on these themes. Building the Network of the Future Internet Internet Platform Platform Internet Internet Platform Shared optical Internet transport Platform Platform Platform Platform Platform Platform Converged Network Core Silo Networks FIGUR