 for- warding table. The forwarding table assigns each packet a new label and directs the packet to an output port. To promote scaling, labels have only local significance. As a result, all packets with the same label follow the same LSP through the network. MPLS traffic engineering enables tunneling through an IP network. With FEC, a group of IP packets are forwarded in the same manner-that is, over the same path and with the same forwarding instructions. LDP refers to the control proto- cols that operate between the LSRs. The LSP is established through a contiguous set of LSRs. The FEC allows treatment of traffic to be based on application, destina- tion, or other parameters. It allows traffic to be grouped together and allows prior- ities to be established. The end result is that the responsibility is split between ingress, egress, and transit routers; heavy processing is done at the edges, and label forwarding is done at the core. Service providers can specify explicit routes by configuring them into edge LSRs manually, or they can use one of two new signaling protocols: RSVP-TE, which is RSVP with traffic-engineering extensions, or MPLS LDP, which is aug- mented for constraint-based routing. Most equipment vendors support both. Virtual Channel Identifier/ IP Packet Virtual Path Identifier Header Packet MPLS Label Received Label Attached Label Packet and Forwarded Forwarded Node 1 Node 2 Figure 10.15 How MPLS works Chapter 10 Next-Generation Networks With MPLS, network operators don't have to use explicit routing-and they are not likely to in networks that have plenty of bandwidth. Instead, they can let ingress LSRs use LDP without any constraint-based extensions, to automatically associate labels with paths. With plain LDP, MPLS packets follow the same routes as ordinary routed packets. With MPLS, you can support all applications on an IP network without having to run large subsets of the network with completely differ- ent transport mechanisms, routing protocols, and addressing plans. MPLS can switch a fram