naling they each have two neighbors along the direction of the LSP. Consider W, which is sending packet B and has two labels: 48 as the topmost label and 72 as the next (and lowest) on the stack. The topmost label (48) is relevant to the next hop in the tunnel. However, the next label (72) is carried through the tunnel until it reaches Z, where it is used. The consequence of this is that W has to participate in signaling with both its immediate physical neighbor and Z. There is no mechanism specified in MPLS to automatically discover peers at the end of tunnels, SO they have to be specified manually (or by some other automatically driven system). In practice an LSP tunnel will have both ends identified on setup through the management function, SO this same function can also configure each end LSR with the peering information. Fast Reroute MPLS traffic-engineering extensions are aimed mainly at a core network provider whose goal is to replace a traditional circuit switched network with an IP/MPLS core. It should be noted that one of the reasons that a pure IP core is not suitable for the replacement of a circuit-switched network is that its recovery time from failure is counted in seconds, whereas circuit-switched networks, such as SONET, can achieve protection switching on the order of tens of milliseconds (50 ms is the often quoted figure). MPLS can provide protection switching on the order of tens of milliseconds using a number of different techniques, as described by RFC 4090 and Marzo (2003). Here only one local repair technique is discussed, and this is termed facility backup by RFC 4090. Facility backup is a local repair technique that provides a backup path to protect a link. The backup path can provide protection for any number of LSPs that pass over the link to be protected; this is attractive to an operator because it does not require individual configu- ration for the many LSPs that might pass over the link. Figure 13.14 shows an example of MPLS facility backup. It assumes that the paths are configured