The key thing to remember about MPLS is that it’s a technique, not a service — so it can be used to deliver anything from IP VPNs to metro Ethernet services, or even to provision optical services. So although carriers build MPLS backbones, the services that users buy may not be called “MPLS”. They could be called anything from “IP VPN” to “metro Ethernet”—or whatever the carriers’ marketing departments dream up next. The fundamental concept behind MPLS is that of labeling packets. In a traditional routed IP network, each router makes an independent forwarding decision for each packet based solely on the packet’s network-layer header. Thus, every time a packet arrives at a router, the router has to “think through” where to send the packet next. With MPLS, the first time the packet enters a network, it’s assigned to a specific forwarding equivalence class (FEC), indicated by appending a short bit sequence (the label) to the packet. Each router in the network has a table indicat