SD-WAN over Segment Routing

SD-WAN over SR | This article explains how SD-WAN over Segment Routing enables a service provider to run per flow services to provide differentiated SLAs to their SD-WAN customers. SD-WAN as overlay technology rides over segment routing for the desired underlay transport treatment. SD-WAN enables customer to mark traffic with desired Qos value , and service provider leverage Qos marking for the classification of traffic per flow basis to provide differentiated treatment to traffic. SR building blocks such as on-demand next-hop, automated steering and Flex-algo are used to accomplish such traffic differentiation and provide SLAs basis customer marking. With existing LDP based MPLS transport, all SD-WAN traffic flows traverse a single path across provider’s transport network.

A brief on SD-WAN (Software defined WAN)

SD-WAN is a Software-defined wide area network overlay technology which spans across geographically dispersed locations. It can connect several enterprise branch locations to central hub office or data center or even public cloud. One of model to deploy SD-WAN where Enterprise can use managed service provider , who owns all networking equipments and maintains them to provide SLAs based services . Service provider in this case give some control to Enterprise to manage Qos policies . These Qos policies or marking helps service providers to differentiate traffic flows when enters their network ( service provider Edge or PE ingress node) .

One of the biggest motivation and selling point behind SD-WAN is the ability to cost-effectively mix and match network links according to traffic type or priority. Internet broadband and 4G LTE/5G are less expensive than MPLS (or SR) , so customers can choose those links instead of the expensive MPLS network for certain types of lower-priority traffic. Even within Service provider network, high priority traffic can traverse certain path, for example low latency path and other traffic can be sent across best effort path (based in IGP shortest path). Please go through basics of SD-WAN here. Also, a good high level article on SD-WAN benefits and summary visit here

SD-WAN over SR

Integration of SD-WAN and SR enables operators to provide transport SLAs to their Enterprise Sd-WAN customers. Here are the high level mechanism to deploy SD-WAN overlay services over segment Routing ,

  1. traffic is marked by customer edge with desired Qos and transport treatment.
  2. Operators leverages Qos marking done by customer edge , transport SLAs are provided using Per-flow ODN and automated steering . Details on ODN and AS , visit here (ODN +AS)
  3. Along with per-flow automated traffic steering, Flex-algo is also used to create non-default IGP path like minimum delay path . More details of Flex-algo visit here (Flex-algo)

Lets understand this with help of example , please refer below sample topology ,

SD-WAN over Segment Routing
SD-WAN over Segment Routing

In the above topology, CE1 and CE2 are SD-WAN customer edge device which are connected to S1 and S4 respectively in the service provider network . S1 and S4 are the PE nodes . In the above topology, links are marked with latency and IGP cost information. Cost of each link is assumed as 10 except link between S2 and S4 which is 100.

Assume, there are two customer’s application , one of them requires lowest latency path to reach between CE1 and CE2 and other requires best effort IGP shortest path. Customer edge device marks different DSCP values to indicate SLA required for these two applications, and operators PE device classifies the traffic flows based on DSCP values. See figure below,

SD-WAN over Segment Routing
SD-WAN Over Segment Routing

Here are the detailed steps followed which allows two different paths for these applications from CE1 to CE2 , one low latency path and other is best effort IGP shortest path. We will use Cisco configuration CLI to illustrate few steps below,

  1. CE1 marks traffic flow with different DSCP values . For example , Traffic for application flow A is marked with DSCP value 46 and traffic for application flow B is marked with DSCP value 8. Application A requires low latency path and application B will send across best effort path.
  2. When traffic received at ingress PE (S1 in this case), S1 match these DSCP values and put into respective forwarding class. Below is the sample configuration ,
Sample Configuration

3. There is MP-BGP between S1 and S4, S1 and S4 exchange L3VPN routes along with color information associated with traffic type A (say color 10) and B (say color 20). If you want more details around the concept , please refer here

4. Nodes S1,S2,S3,S4 and S5 are are participating and configured with Flex-algo, for low latency path with metric as delay (traffic A) . No need to create Flex-algo for best effort path with metric IGP cost (traffic B) as this is default algorithm 0. Below is the sample config needs to be done under IGP (ISIS or OSPF)

Sample Config

5. Now, SR Policy has to be defined for both traffic types as next step , and, map forwarding class 1 with Traffic type A which will use Flex-algo 128 (shown above) . Similarly, map forwarding class 0 with traffic type B which will use default algorithm (IGP shortest path). Below is the sample config for that,

Sample Config

6. In the above config, On-demand Next-hop with color 20 is for traffic Type B which is mapped to forward class 0 , metric used for class 0 traffic is IGP Cost. Similarly, ODN with color 10 for traffic type A which is mapped to forward class 1 will use delay as metric computed using Flex-algo 128.

7. Traffic type A will traverse path as S1-S2-S3-S4 as this is the lowest delay path as compare to any other path (total 20ms) . Traffic type B will traverse S1-S2-S5-S4 as this is lowest cost path.

8. This is how, SR building blocks such as per-flow ODN, Automated steering , Flex-algo can be used to define differential SLA treatment to traffic flows. More details on these concepts can be found here ODN and automated steering.

SD-WAN over SR use case

With Segment Routing network operator can monetize its infrastructure and offer customize per application (per flow) Enterprise SLAs services for SD-WAN customers. To achieve this , as an inherent benefit of segment routing is used which is no per-flow state information is maintained in operator’s network , which means no extra overhead on the network. Operator decides which flow should go via which path. Operators can provide premium SLAs for critical traffic flows and best effort SLAs for less critical traffic .


In Summary , with existing MPLS based transport network, all traffic follows same path, no differential treatment can be given . With Segment routing and its building blocks such as ODN, Automated steering and Flex-algo, SD-WAN traffic flow can be treated differently depending upon the criticality of the traffic . This further provides opportunity for Operators to offer customize SLAs to Enterprise SD-WAN customers. Please visit here if you want see demo of SD-WAN over SR. Also, if you want to be expert on Segment Routing , you can buy these highly recommended books , segment routing part-1 and segment routing part-II

I hope this article is useful . Please provide comments below if you have any query or clarification.

One Comment on “SD-WAN over Segment Routing

Comment Here

%d bloggers like this: