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Dual-plane Disjoint path in Segment Routing

Dual-plane Disjoint paths | In this article we will discuss how Anycast-SID can be used to send different traffic types across dual-plane disjoints path networks. Though, there are other SR based methods to achieve same functionality , one of them is using IGP Flex-algo and other is by dynamic SR policy using SDN controller (SR-PCE) . In today’s discussion , will see how to use Anycast-SID and define Explicit candidate path to achieve the same functionality .

A brief on Anycast-SID , it is a Node Prefix-SID that is advertised by more than one node (typically two or more ). The set of nodes advertising the same anycast-SID form a group called as anycast set. By using an Anycast-SID in the SID list of an SR policy path , it helps achieving traffic load-sharing and resiliency. Apart from this, it can be used to have traffic engineered path through a set of nodes or region (can be called as plane in the network) . More details on this topic can be found here (Anycast-SID in Segment Routing )

Understand Dual-plane vs Single-plane in context with Segment Routing

Dual-plane is the mechanism to enforce disjointness in the network , where , the traffic path towards destination node stays within a set of nodes ( called plane). There can be multiple planes in the network, so that different planes in the network can be. used for different kind of traffic types depending upon the SLA, a service provider signs with customer. Anycast-SID allows creation of such macro policies, such as , “flow of traffic 1 from node A to B must go via plane 1” and “flow of traffic 2 from node A to B must go via plane 2.” This is practically called dual-plane disjoint path architecture.

Example with Sample topology

Lets look at below sample topology to understand how Anycast-SID makes it possible.

dual-plane disjoint path using anycast-sid
Dual-plane disjoint paths using Anycast-SID

In the above diagram, there are two planes , orange plane consists of nodes S2 , S3, S4 and S5. Green plane consists of nodes S6, S7, S8 and S9. Node S1 is ingress and S10 is egress nodes , each of them connects to both planes.

All nodes in orange plane are configured with same Anycast-SID 16100, similarly, all nodes in green plane are configured with same anycast-SID 16200. Below is the sample configuration of Cisco Router for the illustration purpose,

Sample Configuration

Above configuration is common to all nodes in orange plane with prefix-sid as 16100 and to green plane with prefix-sid as 16200. Node S10 is configured with prefix-sid 16110.

Now, in this dual-plane topology, to steer traffic via orange plane towards node S10 consists of SID-list (16100, 16110) and to steer traffic via green plane towards S10 , SID-list consists of (16200, 16110), where 16110 is the node prefix-sid of S10.

So, operator will configure two SR-policies, one for orange plane and other for green plane. Orange plane SR-Policy consists of endpoint which is node S10, color 100 (it can be any value) , which identifies explicit SID list defined for orange plane. Similarly, Green plane SR-Policy consists of endpoint which is node S10, color 200 (it can be any value) , which identifies explicit SID list defined for green plane . More details on SR policy can be found here (SR Policy) and SRTE fundamentals .

Dual-plane disjoint path use case

Assume there is L3 VPN service required between nodes S1 and S10 with two VRFs. These two VRFs should be carried over disjoint paths. Traffic of first VRF should be send across orange plane and traffic of second VRF should be carried via green plane.This can be simply achieved by steering traffic of first VRF into SR-Policy of orange plane , similarly, steering traffic of second VRF into SR-Policy of green plane. More details on traffic steering can be found here (Automated steering).

Conclusion

In Summary , Anycast-SID plays a vital role in dual-plane disjoint path architecture. As i said earlier, dual-plane functionality can also be achieved using Flex-algo and dynamic SR policy using SDN controller (using SR-PCE). For any further detailed explanation and demonstration, you can visit here . 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 helpful , please write comment below for any query or clarification.

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2 Comments on “Dual-plane Disjoint path in Segment Routing

  1. Very well articulated and informative , keep writing

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