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Anycast-SID in Segment Routing

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Anycast-SID | This article explains Anycast-SID and its usage in the context of Segment routing. Anycast-SID in Segment Routing plays a vital role in achieving node resiliency , traffic load-sharing or even to create separate network planes for different types . Recommend to read few basic articles here , what is segment routing and how segment routing works .

Anycast-SID is a Node Prefix-SID that is advertised by more than one node (typically two ). The set of nodes advertising the same anycast-SID form a group called as anycast set. Using an Anycast-SID in the SID list of an SR policy path provides improved traffic load-sharing and resiliency. Few of the benefits of using  Anycast-SID are,

  1. ECMP – Anycast-SID provides traffic load sharing across nodes
  2. High-Availability (HA) – Usage of Anycast-SID provides high availability in case of node failure
  3. Traffic engineering based on specific region or a set of nodes – Anycast-SID is useful when not specific SRTE path is required through the network, path is required through only set of nodes or region, in that case all nodes in that region are assigned same Node Prefix-SID to serve as Anycast-SID to provide load sharing and HA.

Anycast-SID IGP (or BGP) provides shortest path to the nearest node in the anycast set. If two or more nodes are at the same distance (means having same metric) then the flows are load-balanced among the nodes. Lets understand the concept with example, refer figure below for illustration purpose,  

Anycast-SID in Segment Routing
Figure 1 Anycast-SID

Above topology is shown with two domains connected with two border nodes. Anycast-SID 16820 is assigned to border nodes S4 and S5. This is done by configuring same loopback address and same Node prefix-SID  to both the nodes. Now, SR policy is required to reach Endpoint S8 on headend node S1 for a path between S1 and S8. SID-list is consists of (16820,20220). Anycast-SID 16820 load-balance the traffic from Node S1 to Node S4 and S5. By using Anycast-SID on border nodes instead of individual Prefix-SID , the available paths in the network are better utilized using ECMP.

Usage of Anycast-SID provides node resiliency as well , upon failure of node S4, traffic to Anycast-SID is forwarded through remaining Node S5 without changing the SID-list on headend Node S1. Same is true if Node S5 fails and traffic is forwarded through Node S4.

Hence, Anycast-SID provides load-balancing and node resiliency in the SR network. Usage and benefits of Anycast-cast can be applied to use-cases such as disjoint paths and Flex-algo which we will see in the subsequent articles. I hope this article is helpful in understanding Anycast-SID concept. 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

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