DC Fabric Segment Routing Use Case (1)

A couple of weeks ago, I attended a special segment routing Networking Field Day. This set me to thinking about how I would actually use segment routing in a live data center. As always, I’m not so concerned about the configuration aspects, but rather with what bits and pieces I would (or could) put together to make something useful out of these particular 0’s and 1’s. The fabric below will be used as our example; we’ll work through this in some detail (which is why there is a “first part” marker in the title).


This is a Benes fabric, a larger variation of which which you might find in any number of large scale data center. In this network, there are many paths between A and E; three of them are marked out with red lines to give you the idea. Normally, the specific path taken by any given flow would be selected on a somewhat random basis, using a hash across various packet headers. What if I wanted to pin a particular flow, or set of flows, to the path outlined in green?

Let’s ask a different question first—why would I want to do such a thing? There are a number of reasons, of course, such as pinning an elephant flow to a single path so I can move other traffic around it. Or perhaps I want to move a specific small (mouse) flow onto this path, while somehow preventing other traffic from taking it. Given I have a good reason, though, how could I do this with segment routing?

Let’s begin at the beginning. Assume I’m running IS-IS on this network, I have MPLS forwarding enabled, but I don’t have any form of label distribution enabled (LDP, BGP on top, etc.). To make my life simple, I’m going to assign a loopback address to each router in the fabric, and either—

  • just allow IS-IS to use the IPv6 link local addresses (don’t assign any IPv6 address to the fabric links in an IPv6 only fabric)
  • assign some private IPv4 address and configure IS-IS to advertise only passive interfaces, so the fabric link addresses aren’t actually advertised into IS-IS

If you’re uncertain what either of these two options mean, you might want to take a run through my recent IS-IS Livelesson to understand IS-IS as a routing protocol better).

With this background, let’s roll segment routing onto this network. Segment routing, in order to allow many different transports, contains the concept of a Segment Identifier, or a SID. The SID is used for many things, a point which can make reading the segment routing drafts a bit confusing to read. For this particular network, though, we’re going to simplify to two specific kinds of SIDs, because these are the only two we really care about—

These SIDs, in the world of MPLS, are actually just MPLS labels. This means you don’t need a separate form of MPLS label distribution if you’re using the IS-IS segment routing extensions; these labels can be carried in IS-IS itself, along with the topology and reachability information.

To get segment routing up and running, I’ll need each router in the network to create two different MPLS labels, AKA SIDs, and advertise them through IS-IS (using the correct sub-TLV, of course)—

  • An IGP-Prefix segment for each loopback address.
  • An IGP-Adjacency segment for each fabric interface.

This means Router A would create an IGP-Prefix segment for its loopback address, and an IGP-Adjacency segment towards B, F, and its other neighbors.

There is, in fact, another type of SID described in the segment routing documentation, a IGP-Node-Segment. This actually describes a loopback address for a particular node, and hence describes the device itself. This is discussed in section 2.1 of draft-ietf-isis-segment-routing-extensions as a single flag within the IGP-Prefix segment. In reality, there is no functional difference between a node identifier and a prefix identifier in this case, so there’s no need to spend a lot of time on this here.



  1. Jeff Tantsura on 11 July 2016 at 8:06 pm

    Russ – you have been missing a “w” in ISIS URL

    • Russ on 12 July 2016 at 12:44 am

      Jeff — thanks!



  2. NIKHIL on 11 July 2016 at 10:04 pm

    Great article Russ, I am new to segment routing, what I understood with this article is if you do a trace from A-E the top label would be same across all the nodes and that SID is advertised by IS-IS in his TLV’s. Correct me if I am wrong.