ld require a centralized controller.* The controller will have full visibility of the network and can compute explicit paths (if necessary) and push these paths to the router using Path Computation Element Communication Protocol (PCEP) [15]. One of the disad- Distributed control using segment routing is still emerging but it is a hard problem due to lack of getting accurate traffic matrix (trying to create an accurate traffic matrix is nontrivial and would end up losing the benefit of SR by introducing too many counters to track flows) and reservation mechanism to resolve contention when two independent entities trying to grab the same resource. Building the Network of the Future vantages of pure centralized TE is the slow response under failure conditions, especially for span failure where a large portion of traffic gets affected and a quick response is essential in rerouting the traffic without causing congestion. For running a backbone network that is supporting voice, video, mobility, enterprise data, and business critical Internet data, while running the network at high utilization, it is necessary to use a hybrid approach-distributed TE for fast response to failure and centralized TE for global optimization. One way to do this is to use RSVP/TE in parts of the network that requires TE optimization where bandwidth is expensive (intercity core) while considering introducing the new segment routing technology in other parts of the network (metro and data centers). 13.2.4 CORE ROUTER TECHNOLOGY EVOLUTION Traditionally, the core routing function has been fulfilled by large-scale routers that contain custom ASICs capable of supporting over 50 terabits/s of throughput. Recent advances in merchant silicon promise to have sufficient scale and functionality to support core routing requirements. Systems based on these chip sets are beginning to become commercially available. Time will tell if the real- ity lives up to the promise and if these systems can fully displace custom routers. The common fabric introduced in C