Basic concepts

FEC

MPLS groups packets with the same characteristics (such as packets with the same destination or service class) into a forwarding equivalence class (FEC). Packets of the same FEC are handled in the same way on an MPLS network.

Label

A label uniquely identifies an FEC and has local significance.

Figure 1: Format of a label

A label is encapsulated between the Layer 2 header and Layer 3 header of a packet. It is four bytes long and consists of the following fields:

LSR

A router that performs MPLS forwarding is a label switching router (LSR).

LSP

A label switched path (LSP) is the path along which packets of an FEC travel through an MPLS network.

An LSP is a unidirectional packet forwarding path. Two neighboring LSRs are called the upstream LSR and downstream LSR along the direction of an LSP. As shown in Figure 2, LSR B is the downstream LSR of LSR A, and LSR A is the upstream LSR of LSR B.

Figure 2: Label switched path

LFIB

The Label Forwarding Information Base (LFIB) on an MPLS network functions like the Forwarding Information Base (FIB) on an IP network. When an LSR receives a labeled packet, it searches the LFIB to obtain information for forwarding the packet. The information includes the label operation type, the outgoing label value, and the next hop.

Control plane and forwarding plane

An MPLS node consists of a control plane and a forwarding plane.