eal time and it is a single logi- cal entity making routing decisions (as opposed to independent routing decisions by many head-end routers). The packing efficiency of distributed versus centralized routing, is shown with the help of an example in Figure 16.3 [1,6]. The best-possible packing is achieved by the multicommodity flow (MCF) mechanism where each TE tunnel may be split into infinitesimal subcomponents and each such component may be routed in the most efficient manner. We designate the efficiency of MCF as 100% and show the efficiency of the other algorithms in comparison to that. Since distributed routing is done by many different entities, the efficiency is not deterministic and can typically range from 90% to 98%. However, the efficiency of the centralized algorithm used by the SDN controller can usually exceed 99%. Even though the MCF mechanism has the best efficiency, it is not practi- cally realizable since it requires arbitrary splitting of TE tunnels and typically needs a run time in the tens of minutes. In contrast, the centralized algorithm used by the SDN controller is practically realizable and can run in seconds. Distributed Centralized Heuristic MultiCommodity Flow Packing Efficiency X FIGURE 16.3 Packing efficiency of distributed versus centralized routing. Network Measurements 16.2.4 DYNAMICALLY MANAGE AND RECONFIGURE THE MAPPING BETWEEN IP AND OPTICAL LAYERS USING ROADMs Traditional IP networks typically cannot be run at an average utilization greater than 50% (under no-failure condition) for two primary reasons: There is a significant lead time for procuring new IP/optical equipment (typically 3-6 months) and SO essentially we need to build a network that can accommodate almost 6 months of capacity growth. Spare capacity needs to be designed in the network to account for traffic surges and unscheduled failures of single routers and single optical links (fiber spans). Dynamically reconfigurable ROADMs allow us faster provisioning with lead-time cut to just a few weeks. Furthermore, inste