ind an optimal solu- tion, and may leave the network in a state of congestion, even though capacity might be available in other places. Building the Network of the Future Portal Central Tunnel Traffic Management Directed Algorithms Graph Collection Controller storage Plug-in BGP-LS Tunnel Topology routes, State changes Collector Alarms Core CBB Network FIGURE 13.22 Centralized TE architecture. However, centralized solutions are not necessarily robust, because the centralized SDN control- ler needs to be able to communicate to the network to understand the failure, come up with a solu- tion, and distribute that solution to the network. The same failure that needs to be handled by the SDN controller may impact the communication from the controller to the network. The right solution is a combination of both-distributed and centralized control. The distributed RSVP-TE protocols will continue to respond immediately after a failure or traf- fic spike and reroute tunnels to minimize congestion. Capacity management processes will insure that there is enough capacity in the network to protect critical traffic during single node, link, or span failures. The communication from all of the network nodes to the SDN controller will be part of the protected traffic SO this communication should be reliable. However, it is prudent to take a fail-safe approach and design the system SO that in the event that communication to the central controller is lost, the distributed protocols will still be able to route traffic to minimize congestion. Figure 13.22 illustrates the high-level design for centralized SDN control of traffic engineering. Two new protocols are deployed in the network to communicate to the SDN controller. The BGP- link state (BGP-LS) protocol [16] will update the controller on which links are up and how much capacity they have. The path computation element protocol (PCEP) will update the controller on the status of all the MPLS tunnels-their end points, routes, and signaled bandwidth. The exist- ing SNMP protocol will