A Forwarding Equivalence Class (FEC) is a group of packets that are forwarded similarly. A FEC is generally associated with a destination IP network, although it can also be associated with a Layer 2 circuit or an IP precedence value. The Label field of a Multiprotocol Label Switching (MPLS) label is a 20bit field that is used to represent the FEC.

The Forwarding Information Base (FIB) is a table that is built from information in the routing table. When a label switch router (LSR) receives an unlabeled packet destined for an MPLS-enabled interface, it consults the FIB, adds the appropriate label for the destination address, and forwards the packet. However, if the packet's destination address is not contained in the FIB, the packet is dropped.
The Label Forwarding Information Base (LFIB) is a table that contains inbound-to-outbound label mappings. If a route becomes unavailable, the LFIB information will be modified based on information in the Label Information Base (LIB) and FIB tables; the LIB contains all of the labels received from neighboring LSRs. When an LSR receives a labeled packet, it consults the LFIB, swaps or removes the label, and forwards the packet. However, if the label mapping is not contained in the LFIB, the packet is dropped.
A route distinguisher (RD) is a value that enables MPLS virtual private network (VPN) customers to use overlapping IP address ranges; MPLS VPNs are described in Request for Comments (RFC) 4364. An ingress LSR creates a globally unique VPN version 4 (VPNv4) address by adding the RD to the beginning of an IP address. The LSR then assigns a label to the VPNv4 address prefix and stores the inbound-to-outbound label mapping in the LFIB. Authentication to the MPLS VPN is provided based on logical port and RD information.