Packet Forwarding Based on Label Swapping
The packet forwarding process at each router is based on the concept of label swapping. This concept is similar to what occurs at each Asynchronous Transfer Mode (ATM) switch in a permanent virtual circuit (PVC). Each MPLS packet carries a 4-byte encapsulation header that contains a 20-bit, fixed-length label field. When a packet containing a label arrives at a router, the router examines the label and copies it as an index to its MPLS forwarding table. Each entry in the forwarding table contains an interface-inbound label pair mapped to a set of forwarding information that is applied to all packets arriving on the specific interface with the same inbound label.