The use of MPLS in IP networks requires a logical and physical infrastructure consisting of MPLS-capable routers. The labeling process operates primarily within an autonomous system (AS) – a congregation of different IP networks that are managed as a unit and connected via at least one common interior gateway protocol (IGP). Administrators of such systems are generally internet providers, universities, or international companies.
Before the individual paths can be built, the IGP being used needs to ensure that all routers of the autonomous system can reach one another. Then the corner points of the paths, which are also referred to as label switched paths (LSP), are defined. These previously mentioned ingress and egress routers are usually at the inputs and outputs of a system. Activation of the LSPs is then either manual, semi-automatic, or fully automatic:
- Manual configuration: Each note that an LSP runs through needs to be individually configured; this approach is ineffective for large networks.
- Semi-automatic configuration: Only some intermediate stations (for example, the first three hops) need to be configured manually, while the rest of the LSPs receive information from the interior gateway protocol.
- Fully automatic configuration: The interior gateway protocol assumes the entire determination of the path in the fully automatic version; no path optimization is achieved, though.
Data packages sent in a configured MPLS network receive an additional MPLS header from the ingress router. This is inserted between the information of the second and third layers, and is also referred to as a push operation. During the transfer, the individual hops involved exchange the label with a customized version with its own connection information (i.e. latency, bandwidth, and destination hop) – this procedure is often called a swap operation. At the end of the path, the label is removed from the IP header as part of a pop operation.