SRMPLS

SR-MPLS, short for Segment Routing in MPLS networks, is a routing approach that uses the MPLS label stack to encode the path a packet should follow through a network. In SR-MPLS, the path is expressed as an ordered list of Segment Identifiers (SIDs), and the packet is steered by the network as it traverses these segments. This enables source routing without per-flow signaling in traditional MPLS control planes.

The SR-MPLS model relies on SIDs that refer to forwarding instructions rather than fixed next-hops. Key SID types include Node-SIDs, which represent a particular node, and Prefix-SIDs, which map to a specific IP prefix. There are also Adjacency-SIDs that steer packets onto a specific link. The network’s control plane, typically IS-IS or OSPF with SR extensions, distributes SID information so edge and core routers can construct the appropriate label stacks for desired paths. The data plane uses MPLS labels to carry and process these SIDs, with the top label guiding the next action and subsequent labels refining the route or service.

SR-MPLS is used to simplify traffic engineering, enable fast reroute, and support scalable network scaling by reducing signaling complexity compared with traditional MPLS TE (RSVP-TE or LDP). It also facilitates VPN and multi-tenant services by embedding service-level forwarding instructions in the label stack. Deployment requires SR-capable routers and compatible IGP extensions, along with careful planning of SID blocks and provisioning to avoid label conflicts.

See also: Segment Routing, SR-MPLS and SRv6, MPLS, traffic engineering.