MPLS

IP forwarding asks each router to examine a destination address and choose a next hop. Multiprotocol Label Switching, or MPLS, adds another forwarding method. An MPLS router can examine a short label, apply a prepared forwarding action, and send the packet onward.

That description sounds like a shortcut, but speed is not the main reason to learn MPLS. Its lasting value is controlled forwarding. Labels let a network carry different services across shared infrastructure, create explicit paths, stack one forwarding context inside another, and repair selected paths near a failure.

MPLS is not a routing protocol. It is not a VPN by itself. It is not encryption. It is a forwarding architecture that other control-plane protocols and services use.

The core mental model

Think of an MPLS network as an edge that classifies and a core that switches.

unlabeled packet
       ↓
ingress LSR: classify → push label
       ↓
transit LSR: inspect top label → swap label
       ↓
egress LSR: pop label → forward payload
       ↓
unlabeled packet or another labeled service

An MPLS-capable router is a label switching router, or LSR. The ingress LSR assigns a packet to a forwarding equivalence class, or FEC. A FEC is a group of packets that receive the same forwarding treatment. The ingress then pushes a label that represents that FEC.

A transit LSR normally reads only the top label. Its forwarding table maps the incoming label to a next hop and a label-stack operation. The router may swap the top label, pop it, or push another label. The egress LSR removes the transport label and handles the exposed payload or inner label.

This process creates a label switched path, or LSP. An LSP is the path followed by packets in a FEC through one or more LSRs.

Labels are local instructions

An MPLS label is a short, fixed-length identifier. It has meaning in a forwarding context, not across the entire network.

Suppose router P1 advertises label 16020 to its upstream neighbor for a FEC. That value means something on traffic sent toward P1. Another router can use 16020 for a different FEC without conflict. You must read a label together with the interface, platform label space, and direction in which it was learned.

This local significance explains label swapping. A packet can enter a hop with label 16020 and leave with label 24007. The FEC remains the same, but each adjacent forwarding context uses the label expected by its downstream LSR.

The durable troubleshooting question is not “What does label 16020 mean?” Ask instead: