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Introduction to Segment Routing in MPLS Network

Today I am going to talk about the Segment routing. Many of you heard about Segment routing and many of you are experts on Segment routing in Service provider side. The first question which came into mind is what is this Segment Routing? and where we are using this? 

What is Segment Routing?
Segment routing is a method by which the path followed by a packet is encoded in the packet itself similar to lose or strict source routing. The path is encoded as a list of segments, and each segment is identified by the segment ID (SID) consisting of a flat unsigned 32-bit integer. Interior Gateway Protocols (IGP) segments, a sub-class of segments, identify an IGP forwarding instruction.

What are the different groups of IGP Segments?
There are two groups of IGP segments: prefix segments and adjacency segments
Prefix segments: Prefix segments steer packets along the shortest path to the destination, using all available Equal Cost Multi-Path (ECMP) paths.
Adjacency segments: Adjacency segments steer packets onto a specific link to a neighbor.

What are the advantages of the Segment Routing?
We are looking the advantages on the basis of Operational, Scalability and Configuration standpoint, we have the following advantages.
  • From an operational standpoint, it streamlines the operation of an MPLS network by making the label value constant across the core of the network.
  • From a scale and ease standpoint, segment routing is especially powerful in the age of SDN with application requirements programming the network behaviour and where traffic differentiation and engineering are done at a finer granularity.
  • From a configuration standpoint, the number of lines required to enable segment routing is minimum, usually three lines of configuration. 
Fig 1.1- Segment Routing Basics


Can we have a little detail about Segment Routing?
Segment Routing is a network technology that offers enhanced packet forwarding behavior while lessening the requirement for preserving responsiveness of mass volumes of network state. 

Segment Routing contents crucial necessities for application-enabled routing in software defined networks, including the capability to deliver strict network performance guarantees, well-organized use of network resources and very high scalability for application-based transactions. 

Segment Routing trusts on a small number of extensions to Intermediate System-to-Intermediate System (IS-IS) and Open Shortest Path First (OSPF) and can drive with an MPLS or an IPv6 data plane. 

It incorporates with the rich multi-service capabilities of MPLS such as Layer 3 VPN (L3VPN), Virtual Private Wire Service (VPWS) and Virtual Private LAN Service (VPLS) / Easy VPN (E-VPN). It’s an establishment for application engineered routing because it prepares the networks for new business models where applications can direct network behaviour. 

Segment routing requests the right balance between distributed intelligence and centralized optimization and programming. It was built for the software-defined networking (SDN) era.

A segment can have a local semantic to a segment-routing node or global within a segment-routing network. Segment routing allows you to enforce a flow through any topological path and service chain while maintaining per-flow state only at the ingress node to the segment-routing network. 

To be aligned with modern IP networks, segment routing supports equal-cost multipath (ECMP) by design, and the forwarding within a segment-routing network uses all possible paths, when desired.