As described in draft-ietf-idr-bgp-ls-flex-algo-06, although link IGP metrics are often used to determine the best paths across a network, deployments that use RSVP-TE or SR-based TE require different metrics to determine the best path; these networks can use the SR Flex-Algorithm to determine the best constraint-based paths. This section describes the use of SR prefix SIDs to compute a constraint topology and send packets along the constraint-based paths.
Using Flex-Algorithms can reduce the number of SR SIDs that must be imposed to send packets along a constrained path; this implementation simplifies the hardware capabilities of SR routing tunnel head-end devices.
The supported depth of the label stack is considered in an SR network when SR-TE tunnels or SR policies are deployed. In such tunnel policies, the packet source routing is based on the SR label stack pushed on the packet. The depth of the label stack that a router can push on a packet determines the complexity of the SR-TE tunnel construction that the router can support.
The SR Flex-Algorithm solution allows the creation of composed metrics based upon arbitrary parameters (for example, delay, link administrative group, cost, and so on) when using Flex-Algorithms. A network-wide set of composed topology constraints (also known as the Flexible Algorithm Definition (FAD)) creates an SR Flex-Algorithm topology. The IGP calculates the best path using constraint-based SPF and the FAD to create the best paths through the Flex-Algorithm topology.
With Flex-Algorithms, each Flex-Algorithm topology can send data flows along the most optimal constrained path toward its destination using a single label, which reduces the imposed label stack along the constrained path.
Using this solution, backup path calculations (for example, Loop Free Alternate (LFA), Remote LFA (R-LFA) and Topology Independent LFA (TI-LFA)) can be constrained to the SR Flex-Algorithm topology during link failure.
The procedures in subsequent sections describe how to configure Flex-Algorithms using IS-IS.