DC Fabric Segment Routing Use Case (5)

In this, the last post on DC fabrics as a Segment Routing use case, I mostly want to tie up some final loose ends. I will probably return to SR in the future to discuss other ideas and technical details.


Anyone who keeps up with LinkedIn knows anycast plays a major role in many parts of the infrastructure. This isn’t unique to LinkedIn, though; most DNS implementations and/or providers, as well as just about every large scale public facing web application, also uses anycast. Which leads to an obvious question—how would SR work with anycast? The answer turns out to be much simpler than it might appear. The small diagram below might be helpful—


Assume A and B have two copies of a single service running on them, and we want hosts behind F to use one service or the other, just depending on which the routing system happens to route towards first. This isn’t quite the classical case for anycast, as anycast normally involves choosing the closest service, and both of the services in this example are equal distance from the hosts—but this is going to be the case more often than not in a data center. In section 3.4 of draft-ietf-spring-segment-routing, we find—

An IGP-Anycast Segment is an IGP-prefix segment which does not identify a specific router, but a set of routers. The terms “Anycast Segment” or “Anycast-SID” are often used as an abbreviation. An “Anycast Segment” or “Anycast SID” enforces the ECMP-aware shortest-path forwarding towards the closest node of the anycast set. This is useful to express macro-engineering policies or protection mechanisms. An IGP-Anycast Segment MUST NOT reference a particular node. Within an anycast group, all routers MUST advertise the same prefix with the same SID value.

To advertise the anycast service running on both A and B, then, a single label can be assigned to both, and advertised as an Anycast-SID through the entire network. When F receives both, it will treat them (essentially) as a unicast route, and hence it will choose one of the various paths available to reach the service, or (perhaps) load share between them (depending on the implementation). The ideal implementation in a data center fabric would be to load share between the two advertisements.

Strict Shortest Path Flag

In section 3.2 of draft-ietf-isis-segment-routing-extensions we find—

Strict Shortest Path First (SPF) algorithm based on link metric. The algorithm is identical to algorithm 0 but algorithm 1 requires that all nodes along the path will honor the SPF routing decision. Local policy MUST NOT alter the forwarding decision computed by algorithm 1 at the node claiming to support algorithm 1.

What is this about? At any particular hop in the network, some local policy might override the result of the SPF algorithm IS-IS uses to calculate shortest (loop free) paths through the network. In some situations, it might be that this local policy, combined with the SR header information, could create a forwarding loop in the network. To avoid this, the strict flag can be set so the SPF calculation overrides any local policy.

Using SR without IGP distribution

The main problem an operator might face in deploying SR is in implementing the IGP and/or BGP extensions to carry the SR headers through the network. This could not nly expose scaling issues, it could also make the control very complex very quickly, perhaps offsetting some of the reasons SR is being deployed. There are several ways you could get around this problem if you were designing an SR implementation in a data center. For instance—

  • Carry SR headers in gprc using YANG models (you should be thinking I2RS about now)
  • Carry SR headers in a separate BGP session to each device (for instance, if you use eBGP for primary reachability in the fabric, you could configure an iBGP session to each ToR switch to carry SR headers)

There are many other ways to implement SR without using the IGP extensions specifically in a data center fabric.

SR is an interesting technology, particularly in the data center space. It’s much simpler than other forms of traffic engineering, in fact.