l is fully imple- mented, including programming of new rules to the forwarding tables of all intermediate nodes, before moving the traffic. It does this by verifying that an in-band probe, that uses the same label swapping machinery of the LSP that user traffic will use, can be looped back through the tail-end back to the head end. This make-before-break approach, which is enabled through the use of signaled tunnels, provides completely hitless rearrangement capabilities. When the link is repaired, the head end is notified through the routing protocol, it will determine the optimal route is via A-B-C-D (as was the case originally before the failure), signal it, verify it, and hitlessly move traffic back to the new tunnel. The hitless rearrangement is not only used for reoptimization after a failure, it is also used during planned events. AT&T deploys two core routers in each office. When a maintenance event is per- formed on one router, it is first "costed out." This is accomplished by increasing the metric assigned to each of its links causing all head ends with tunnels traversing this node to recompute new paths for their tunnels that avoid this node. Hitless rearrangement makes this a completely nonintrusive process. 13.2.2.3 Distributed Traffic Engineering: Bandwidth Aware Routing The implicit assumption is that there is sufficient bandwidth available along the reoptimized route to support not only the traffic that was there but also the rerouted traffic. It is common policy to deploy additional capacity in the core above and beyond what is needed to support user traffic demands during sunny day scenarios to be available when capacity is temporarily lost due to fiber cuts or equipment failures. We loosely refer to this as restoration capacity. In AT&T's case, the network is engineered to protect 100% of the critical traffic during all possible single link (the loss of all bandwidth between a pair of adjacent nodes), span (the loss of all facilities that share common fiber routes), and complete node failures. 