The fundamentals of Segment Routing


Do we have to buy a new network equipment?

Do we need to pay more on HW resources?

How do I migrate my current network to Segment Routing based network?

Can we use the current running L2/L3 services with the SR based network?

What is the benefit of Segment Routing?

These are some of the most commonly asked questions by the customers – network operators. In this article, I’m starting with the basics of Segment Routing, the Building blocks of SR, and closing this part by answering the questions above.

Segment Routing is a new way of Label distribution within a single or multi IGP/BGP domain. Based on many Drafts published by IETF so far, simply by extending current Link State protocols (OSPF & IS-IS), now we have the opportunity to distribute Labels without any additional protocol as previously (LDP or RSVP). That means, using only an IGP protocol we are given the opportunity to distribute MPLS labels, in case of IS-IS additional TLV-s are used to distribute the Label/Prefix/Link information, and in case of OSPF additional Opaque-LSA is used to distribute the same information to other Nodes on an IGP domain.

As for BGP, I’ll park this one for an upcoming article, in order to get the understanding of the IGP Based SR first, so that it gets easier later on to move with BGP based SR.

A fundamental Segment Routing construct, called the Segment (a label) or Segment ID (SID), is a representation of an instruction, either topological or service based.

This instruction needs to be understood by all the other nodes within the same domain where SR is applied.

Two type of Segments exist today:

  • Prefix SID
  • Adjacency SID

Prefix SID (sometimes called a Node-SID) is a unique global label value within the IGP domain. Once an IGP protocol starts advertising prefixes to other nodes, it advertises an unique Prefix SID (user configurable) the same time. This is important because the rest of the Nodes now have the capability to forward traffic based on that unique label value that doesn’t change from hop to hop (as it used to be with LDP), but instead the packet is processed based on MPLS Top label, which in this case is the Prefix SID, and swapped along the way to destination with the same unique label value (this is also a good use case for traffic capacity planning).

Adjacency SID is a locally significant label value assigned by the node itself to all IGP enabled physical links connected to other nodes. This means, none of the other directly connected neighbors will accept nor install the Adjacency SID of other nodes in its own FIB/LFIB. The Adjacency SID is not configurable, it is something the Node itself assigns automatically at the moment you activate SR under an IGP protocol. One of the use cases for Adj SID is Traffic Engineering, and the other use case is Fast Reroute also called TI-LFA.

Below is an example of Prefix-SID and Adjacency-SID:


Similar to LDP based MPLS networks where you have a range of labels reserved for LDP based distribution, with Segment Routing we have the so-called Segment Routing Global Block (SRGB).

SRGB reserved range of label values is 16000 – 23999. Prefix-SID and Adjacency-SID assign a label value to Prefix/Link from the pool of SRGB.

It is recommended that in a multivendor network environment keeping the same SRGB range, in order to avoid any potential issues.

Let’s now look at how SR based MPLS forwarding is done:


We still use MPLS date plane when it comes to forwarding, what is different here with SR is the Control Plane, now instead of LDP/RSVP we use SR (OSPF/IS-IS extensions).

To answer the questions in the beginning, there is no need for a new Hardware as long as your software supports the extended version of OSPF/IS-IS, which typically by now most of the vendors have that capabilities on their software.

Migration of LDP based to SR based Core/Transport network is as easy as turning on SR in no particular order, at the same time you have LDP running, that means the Router will prefer LDP based labels for forwarding. Once all nodes distribute SR labels, you add an additional command on the IGP level, preferring Segment Routing for forwarding now, without any impact or performance issues on the data plane.

All L2/L3 services running currently on LDP/BGP can still run without any changes to the configuration with the new SR based Core/Transport network.

There are many benefits running SR, one of them being able to offload the Control Plane to a SDN Controller, creating end-to-end LSPs between different domains without having full visibility, and less overhead because now it is the IGP that distributes the Labels instead of having LDP/RSVP do that in the traditional old fashioned way.

This article is just a scratch on the surface of Segment Routing topic, if you think you want to know more in details I suggest you read the following Draft, as it explains the bits and bytes of SR:

Segment Routing Architecture.

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