Segment Routing Overview


Segment Routing (SR) is a source routing paradigm relying on the Source Packet Routing in Networking (SPRING) architecture.

Segment Routing Overview

  • Segment identifies an instruction for a router to take. One example of such instruction is to forward packet thought a defined node or a defined adjacency.
  • SR defines a path, which is an ordered list of segments. Similar to source based routing for IPv4.
  • Each segment is identified with SID (Segment ID), which consists of a 32 bit integer.
  • There is a number of different segment types. The most common is the IGP Segment which is made up of two groups: prefix segments and adjacency segment.
    • Prefix Segments - steer packets to the destination network.
    • Adjacency Segments - steer packets onto a specific link to a neighbors.
  • SR is thought to be the replacement of MPLS TE, but without using RSVP protocol and maintaining tunnel states at each node of the backbone.
  • Per Flow state kept only on ingress SR edge node, which pushes the segment on the packet.
  • SR eliminates the need for a label distribution protocol (LDP and RSVP or BGP). The functionality is replaced by segments via Link State IGP protocols.
  • SR allow for ECMP traffic engineering.
  • There are two data plane applications of segment routing: MPLS and IPv6.
    • For MPLS forwarding, a segment is represented in a label.
    • For IPv6 forwarding, a segment is represented in a IPv6 routing extension header
  • Control plane of segment routing is mostly performed with either IGP or BGP. Protocols have extensions to carry segment information.
  • Below are the required protocol extension:

Additional Resources

Segment Routing Architecture - draft-filsfils-spring-segment-routing-04


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