LS packets solely based on the tunnel mapping. In case of a link failure, two mechanisms come into play. First, every link has a predefined backup reroute path to its neighboring core routers. Second, as the reachability infor- mation is propagated across the network, all of the tunnels redetermine their paths SO that they no longer expect to use the now failed link. This global repair allows for better global optimization of core capacity. Using SDN, all global traffic information and link state can be accumulated in the SDN controller for even better global optimization. This allows further tuning of the network. This hybrid approach of using both distributed and centralized control was selected to best balance speed of initial restoration with maximum efficiency. In the near term, a new approach called seg- ment routing (or SR), is being introduced. SR allows simplification of the network since multiple control plane protocols can be consolidated. Essentially SR works by allowing each packet to carry path routing information. Thus, at the origination point, some or all of the intermediate hops can be predetermined. Thus, SR supplants the need for separate protocols such as Resource Reservation Protocol for Traffic Engineering (RSVP-TE) to execute fast reroute (FRR) local repair and label distribution protocol (LDP) for label distribution. This is discussed in more detail in Chapter 13. The final component of the core is the route reflector (RR). All of the edges that connect to the core need the ability to communicate reachability of service with each other. This is done using BGP. But how do you let hundreds of edge routers communicate control plane information with each other? By using an aggregation and distribution node known as a RR. Every edge router in the network connects to a pair of RRs. For scale, multiple pairs of RR can be deployed for differ- ent services or geographic regions. Each of these take in the BGP messages from the edge routers they serve and replicate and distribute the messages to the