d that the destination is C. Then the packet will be examined a second time at C, and the topmost label will be swapped to 61. However, in practice it can be known that C will perform this pop operation and two lookups for every packet entering the backup LSP. Thus, it is possible for the pop operation to be performed at F and the lowest-level label is then exposed directly to C for forwarding onto the original tail end of the LSP. This reduces the workload on the egress LSR and is generally used at an LSP tunnel egress. Summary of Traffic Engineering The TE features of MPLS are one of the strongest reasons for deploying it, because it offers facilities that are difficult or impossible to replicate using conventional IP. In particular, MPLS MPLS Virtual Private Networks can provide traffic trunks that can replace the overlay systems used with circuit-switched networks. This has advantages because the MPLS architecture is designed to be compatible with IP, whereas an overlay gives rise to the problems discussed earlier. It should be noted that MPLS TE systems can (and probably will) coexist with LDP within the network. Additionally, while there will be a path configuration system for MPLS TE tunnels, this needs to coexist with a suitable Interior Gateway Protocol (IGP) that will configure the conventional IP routing tables such that IP connectivity is provided for signal- ing and Operation, Administration, and Maintenance (OAM) requirements. MPLS Virtual Private Networks The second major application for MPLS described in this chapter is to provide a layer-3 VPN. This type of VPN provides a replacement for a leased-line service but with some controlled layer-3 interaction between provider and customer networks. This service is the most popular reason that operators have deployed MPLS-because they need a replacement for the ATM and Frame Relay services they were offering to customers. One motivation for MPLS compared to ATM and Frame Relay is that MPLS has been designed to support IP. Some of the arguments supportin