nts on the SDN controller: For the purpose of reliability and disaster recov- ery, there should be one active SDN controller and one or more standby SDN controllers. The status of each of the SDN controllers should be continuously monitored and if it is determined that the active controller is unable to control the network, one of the standby controllers should become the active controller. Building the Network of the Future It should be noted that while some of the above data are generally available today, in the pre- SDN environment, it is not used proactively for real-time traffic management in a coordinated and centralized manner. In the SDN environment and through the controller managing the routers, ROADMs and other network elements along with efficient optimization algorithms, one can use these real-time data to optimally manage the transport network maintaining the highest level of reliability and to allow ISPs to provide new services. While some of these techniques were referred to in Chapter 13, a few examples of the use of real-time traffic measurements in an SDN network are given below. 16.2.3 CENTRALIZED TE USING SDN CONTROLLER TO MORE EFFICIENTLY MANAGE TE TUNNELS The traditional routing of TE tunnels uses a distributed mechanism in which each TE tunnel is routed by the head-end router. Each head-end router knows the spare bandwidth available on each IP link and the path and bandwidth used by tunnels controlled by the router but it does not have a global view of the path, and the bandwidth, of all tunnels. Furthermore, two different head-end routers may try to access the same spare bandwidth at an IP link to route their tunnels. This may result in inefficient routing. In order to eliminate this inefficiency, we use the end-to-end measure- ments (edge-to-edge or core-to-core) and the individual IP link measurements mentioned above, and feed that to the SDN controller. The routing inefficiency is eliminated since the SDN controller has the global view of the path and the bandwidth of every tunnel in r