 using RSVP-TE and that the backup path is preconfigured SO that there is no delay due to path computation or signaling. Figure 13.14(a) shows one LSP in normal operation over the path A, B, C, D. The local repair path B, E, F, C is used to protect link BC and is set up using RSVP-TE before the failure occurs. Before the point of failure no traffic is passed into the backup LSP (although the links on the backup path can carry other LSPs). The facility backup method makes elegant use of the label stack to provide the backup path such that switching is only required at the head end of the protected link, which is LSR B. Note that in normal operation B puts label 23 onto traffic destined for C on the LSP. When a failure occurs on link BC, B detects this quickly from the physical layer and initiates the Chapter 13 Multiprotocol Label Switching Fig. 13.14 MPLS fast reroute: (a) the LSP in normal operation; (b) the backup LSP in use following a failure on link BC. Note that the backup LSP makes use of a label stack. head-end protection switching. All this requires is the labeling of packets with 23 (as before) and the tunneling of the packets through the backup path B, E, F, C, as shown in Figure 13.14(b). To pass the packet into the backup path tunnel only requires B to push label 71 onto the packet and output to E. The backup LSP is configured such that when the packet reaches C the top-level label has been popped, revealing label 23, as was used before the protection event. There is one point shown in Figure 13.14(b) that applies generally to LSP tunnels but has not been described yet; this is the ability of LSRs to perform Penultimate Hop Popping (PHP). This can occur on the LSR immediately before the end of the tunnel. In Figure 13.14, F is the penultimate hop before the end of the backup tunnel at C. Following the earlier discussion of LSP tunnels, it would be expected that as the packet enters C (on the backup LSP) it has two labels on it. The first will be examined and the result will be that a pop is needed an