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Segment routing

Segment routing definition

Segment routing is a network routing method where data packets are forwarded along a path specified by the source node (the originating router or edge device). This path is divided into segments — individual links along the route with a unique Segment Identifier (SID). Segment routing is the modern version of source routing.

See also: firewall, load balancer, intrusion detection system, generic routing encapsulation, internet routing, routing information protocol, routing table, static routing

How segment routing works

The source node encapsulates a data packet with a list of SIDs that represents the explicit path it should take through the network. Having a complete path ready from the get-go reduces the need for complex routing protocols and calculations in intermediate routers.

Each router along the path reads the next SID from the list and forwards the packet to the corresponding segment. The packet is thus 'steered' through the network based on the SIDs in the encapsulation header, without the need for additional routing lookups at every hop.

Advantages of segment routing

  • Network operators can fine-tune network paths to optimize network resources — for example, they can plan for latency, bandwidth, and network congestion.
  • Pathing decisions are made at the source node, reducing the need for complex protocols and maintaining simplicity at the core of the network.
  • Segment routing is not dependent on specific network topologies. It can be deployed in various network architectures, including MPLS (Multiprotocol Label Switching) and IPv6. In SR-MPLS, the SIDs are labels, and in SRv6, the SIDs are IPv6 addresses.
  • A packet may be routed through multiple network services (an act known as “service chaining”) in a predetermined order, helping services like network firewalls, load balancers, and intrusion detection systems perform their function.