What is SR?
SR (Segment Routing) is an IP-based routing architecture that forwards packets along a path dynamically defined at the source node by inserting an ordered list of instructions (segments) into the packet header. SR simplifies network operations by eliminating the need for per-flow state in intermediate routers, and is widely adopted in 5G transport networks (backhaul, midhaul, fronthaul) for traffic engineering, network slicing isolation, and service chaining.
How Does SR Work?
In SR, the source node prepends a list of Segment Identifiers (SIDs) to the packet header. Each SID represents an instruction — either a topological instruction (forward to a specific node or via a specific link) or a service instruction (apply a specific function). Intermediate nodes process the active SID, forward the packet accordingly, and advance to the next SID. SR can operate over MPLS (SR-MPLS, using MPLS labels as SIDs) or over IPv6 (SRv6, using IPv6 addresses in a Segment Routing Header). SRv6 is particularly relevant for 5G because it natively supports network slicing — each slice can be assigned a distinct SRv6 SID, ensuring end-to-end path isolation without complex per-flow MPLS tunnels.
Use Cases
5G transport network traffic engineering, end-to-end network slicing in the transport domain, service function chaining for MEC and security services, fast reroute for transport resilience, and programmable transport for SDN-based network management.
3GPP / Standards Reference
IETF RFC 8402 (Segment Routing Architecture), IETF RFC 8986 (SRv6 Network Programming), 3GPP TS 23.501 (5G transport network considerations)
Related Terms
SDN | Network Slicing | FlexE | Midhaul | Core Network
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This glossary entry is part of the 5GWorldPro Complete 5G Glossary. To go deeper into 5G architecture and technology, explore our 5G Training courses.
