What is Segment Routing?

Segment Routing (SR) is a modern source-routing paradigm where the source node (or SDN controller) specifies the entire path through the network by encoding it as an ordered list of segments (instructions) in the packet header. Each segment represents an instruction — "go to this node," "take this specific path," or "apply this service function." This eliminates the need for per-flow state in intermediate nodes (no RSVP-TE tunnel state). Two data planes: SR-MPLS — uses the existing MPLS label stack; segments are encoded as MPLS labels. SRv6 (Segment Routing over IPv6) — uses IPv6 extension headers (Segment Routing Header/SRH) with 128-bit SID (Segment Identifier) values derived from IPv6 addresses. Benefits: simplifies MPLS networks (no LDP/RSVP), enables flexible traffic engineering, supports TI-LFA (fast reroute), and integrates naturally with SDN controllers. Used by major ISPs and cloud providers for inter-datacenter traffic engineering. Cisco, Juniper, and Nokia all support Segment Routing.