down the root cause of the failure and thus speed up troubleshooting. The wavelength circuit infor- mation is also important for capacity planning. The routing and SRLG data are used to determine additional resources needed under each potential failure scenario. 16.3.6 LAYER 0 AND LAYER 3 RESOURCES In traditional networks, both layer 0 and layer 3 are static in that adding and removing connections are usually performed manually. An SDN-enabled optical network facilitates dynamic service pro- visioning in that a wavelength circuit can be programmatically established or released on demand, thus changing the layer 3 network in a matter of minutes. Such dynamic provisioning can be used to establish new wavelengths to add capacity to the network on the fly. It can also be used to perform failure restoration. Figure 16.6 shows three cases that leverage optical layer restoration. In case (a), a line card failure on router1 disconnects routerl and router2. The standby connection between router1 and ROADM1 is used to reconnect the two routers. In case (b), a failed fiber disconnects router1 and router2. At both ends, the line card-transponder connections become idle. The SDN controller can reuse such resources to make a new wavelength along the alternative path, where an Building the Network of the Future ROADM1 ROADM2 ROADM1 ROADM2 ROADM3 ROADM3 ROADM1 ROADM2 ROADM3 FIGURE 16.6 Optical layer dynamic restoration (dotted lines are broken lightpaths, and thick lines are restored lightpaths). (a) Port failure: Switch to standby port, (b) Fiber failure: Switch to alternative lightpath, (c) Router failure: Set up new lightpath between R1-R2. idle wavelength is available. In case (c), router3 fails completely. The SDN controller configures ROADM3 to reuse the released wavelengths and transponders to add new capacity between router1 and router2, which helps to deliver traffic previously traversing through router3. An essential requirement to perform dynamic provisioning is to have complete and accurate description of layer 0 and