Flex-algo and SRTE |In my earlier articles , we have discussed SR-TE and Flex-algo in silos, now we will discuss both together how Flex-algo can be used in SR traffic engineering. Please refer to below links for Flex-algo and SR-TE articles.
Flex-algo is integral part of SR-TE functionality. Flex-algo and SRTE together compliments each other and Flex-algo enriches SR-TE functionality to provide SLA paths and , is fully integrated with other SR-TE mechanisms such as Automated steering and On-demand Next-hop (ODN).
Lets understand how Flex-algo combines with SR-TE with below example topology, refer figure 1 below,
By default IGP algorithm 0 is enabled on all nodes which uses IGP metric as cost of the links and assume algo-130 is configured with metric definition as delay and enabled on all nodes shown in the topology. IGP metric for all links is 10 except link between node S2 to S3 which is 100 (cost of the links) and delay of each link is shown above against each link.
When defining SR policy with optimization metric as IGP (cost) to compute dynamic path from Node S1 to Node S4, computed path is going to be S1-S2-S5-S4 as lowest cost path.SR-TE would encode this path as SID list 16004 only which belongs to default algorithm 0.
When defining SR policy with optimization metric as delay to compute dynamic path from Node S1 to Node S4, computed path is going to be S1-S2-S3-S4 as lowest delay path. SR-TE would encode this path as SID list 16304 only which belongs to Flexible algorithm 130.
Flex-algo SID allows to express path as single SID instead of stack of SIDs.Encoding Flex-algo SID by SR-TE provides benefits in terms of scalability (SID list scale) and resiliency. Even upon failure of the link , SR-TE does not have to update SID list because IGP will update the path to Prefix-SID (to 16004 or 16304).
Moreover, TI-LFA will compute port-convergence back-up path(with delay metric) for Flex-algo SID 16304 and computes back-up path (with metric cost) for Flex-algo Prefix-SID 16004.
In Summary, any node participating in Flex-algo computes the paths to the prefix-SIDs of that Flex-algo. I hope this article will help understanding how Flex-algo compliments SR traffic engineering .