a fault in the network. The user should not see any disturbance to his or her data traffic and should not have to take any remedial action. Chapter 8 MPLS offers many options for protecting LSPs from network problems. In optical networks, these procedures utilize some specific extensions to MPLS, which are described in the following section. Notify Messages Protection switching (as shown in Figure 8-4) is a scheme where a network failure is recovered by directing data from an interrupted primary LSP onto a typically presignaled backup LSP that uses a disjoint path. In Figure 8-4, black and gray lines represent primary LSPs through the network. When the shared link in the middle of the network fails, data is redirected to flow on the backup LSPs (represented by the dashed lines). In order for this to work, an upstream node-the repair point-must become aware of the error. The repair point is usually the ingress/initiator, and in nongeneralized MPLS, the notification method is the PathErr message, which flows upstream following the path of the LSP. This message flow is effective, but is slow because of the following reasons: The path selected for the LSP may not be the shortest point between the detection point and the initiator (because of the traffic-engineering routing of the LSP), but the PathErr still follows this path delaying the notification of the error. Figure 8-4 Protection switching with Notify messages. Primary LSPs Backup LSPs Notify Messages GMPLS in Optical Networks A PathErr message is intercepted and (minimally) processed at each LSR that it passes through, which impacts the time taken to notify the initiator of the problem. There may be many affected LSPs, all of which need PathErr messages to be sent at the same time-this can clog the network and consume important resources at the LSRs. It is possible that an upstream node receiving the PathErr could incor- rectly decide to take responsibility for the error, try to initiate local repair itself, and not forward the PathErr upstream until this had 