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Flex-algo enables creating custom IGP path computation algorithm which calculates best path basis non-default metric like delay, cumulative latency and TE-metric or even link affinity (means to include or exclude particular links) etc (IGP like ISIS/OSPF uses cost as default metric to calculate best path as part of SPF algorithm).
Lets be very clear that SRTE (Segment Routing traffic-engineering) is not an alternative to Flex-algo but rather Flex-algo compliments SRTE . Combining the benefits of SRTE like ODN (on-demand Next-hop policy) , per destination or per flow automated traffic steering with Flex-algo creates end-to-end Network Slicing in the Transport network.
Each node advertises Flex-algo information via IGP (ISIS/OSPF) as an additional prefix-SID and computes shortest-path tree and topological database with the metric defined in Flex-algo . Each Flex-algo topological database is logically independent from each other and also runs independent TILFA (Topology independent loop free alternate) for fast-reroute during link and node failure .
Lets understand it better with following example,
In the above sample topology , say traffic type A requires a low latency path and Traffic type B requires to be sent across default path. Flex-algo with definition (metric ) as delay can be created and traffic type A is steered across path in the network calculated basis lower delay . Similarly, traffic type B is sent over the path uses default algorithm which is SPF with metric as cost.
There are many use case to use Flex-algo , some of them are
Network Slicing – Low latency slice for uRLLC traffic from 5G context
Enterprise VPN to steer different traffic flows through different paths
SD-WAN over Segment routing with differential flow treatment
To avoid few links in the network for specific traffic (link affinity)
Flex-algo will further evolve for many other use cases in future . I hope this article was useful.