It has been around for some years now and we are seeing quite many deployments with it. Segment Routing, with either MPLS or IPv6 data plane, solves a lot of issues in Networking, mainly scalability but also it comes with additional benefits that is not quite possible with other network technologies. 

In this post, we will briefly touch on what is Segment Routing and what it is doing in the Networking industry.

Segment Routing (SR) is a protocol, designed to forward data packets on a network based on source routes. 

Segment Routing divides a network path into several segments and assigns a segment ID (SID) to each segment and forwarding node. 

The segments and nodes are sequentially arranged into a segment list to form a forwarding path.

You can get a very well and detailed course about Segment Routing, read more here.

Segment Routing comes with two different data plane options

Segment Routing is divided into two types, based on the forwarding plane.

Segment Routing MPLS (SR MPLS for short) is based on the MPLS forwarding plane, whereas Segment Routing IPv6 (SRv6 for short) is based on the IPv6 forwarding plane.

It is an alternative to LDP and RSVP which means without using the LDP and RSVP we still can generate and distribute the transport labels and steer the traffic without using the RSVP signaling.

Segment routing is based on label switching but for label generation and distribution, LDP and RSVP are not used. Segment Routing is an extension of an IGP (OSPF/ISIS). Labels are called segments in Segment Routing MPLS

There are many use cases with Segment Routing 

Segment Routing Traffic Engineering, Segment Routing for MPLS Layer 2 and MPLS Layer 3 VPNs, Segment Routing Fast Reroute – Topology Indendepnt Loop-Free Alternate – TI-LFA, and using Segment Routing. for OAM purposes are common deployment use cases today in Networking Industry.

MPLS VPN for example can be set up by using Segment Routing. In order to create an end-to-end MPLS VPN service, Segment routing and MPLS VPN have to be provisioned in the network together.

So, Segment Routing is Underlay, MPLS VPN is Overlay. Please note that Segment Routing eliminates to use of LDP for the transport label operation only.

If you set up MPLS layer 2 VPN service and use LDP for PW signaling, Segment Routing and Targeted LDP are used as two control plane protocols to setup MPLS L2 VPN. There are so many things to know about this technology and if you are working in the Service Provider or medium to large enterprise organization, you should definitely learn this technology well. Orhan Ergun is providing a very detailed video-on-demand training course for it that we recommend you to check.


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