Flexible Algorithm for ISIS with SRLG

Overview

Flexible Algorithm allows Interior gateway Protocols (IGP) to calculate additional best paths based on specified constraints, thereby providing simple traffic engineering without using a network controller. Shared Risk Link Group (SRLG) provides an indication of which links in the network might be at risk from the same failure, and allows specifying such links to be excluded during Flex-Algo path computation.

Shared Risk Link Groups (SRLG) Disjoint Backup Path: This feature protects traffic against failure of a group of links that share the same risk of failure. This allows the setup of disjoint path(s) for each specific default-Algo or Flex-Algo by leveraging deployed SRLG configurations. It facilitates resiliency against failure of any links along the primary path(s).

This is an enhancement to Topology Independent Loop-Free Alternate (TI-LFA) based calculations of backup path(s) that provides fast reroute protection (FRR), minimizes downtime, and provides re-convergence on failure detection.

Flexible Algorithm Exclude SRLG Constraints: This feature computes flexible algorithm specific path(s) that avoid links with the same risk that an operator intends these path(s) to avoid. When Exclude SRLG Constraints are configured for a Flexible Algorithm definition, the computed path(s) do not transverse any links that advertise the same SRLG(s) as included in the specific Flexible Algorithm definition.

The Exclude SRLG constraints based link pruning only applies to Flexible algorithm specific path(s), and to both primary and backup path(s) during Constrained Shortest Path First (CSPF) base path computation procedures.

Feature Characteristics

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Multiple Flex Algo(s) can be defined by excluding different sets of SRLGs. Each Flex-Algo prunes the links belonging to the excluded SRLGs from its topology on which it computes its paths.

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Provides a new alternative to creating disjoint paths for Flex-Algo, in addition to leveraging the link’s admin-group, extended-admin-group, or maximum delay constraints as Flex-Algo constraints.

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When computing the shortest path tree for particular Flexible Algorithm:

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All nodes that do not advertise support for such Flexible Algorithm are pruned from the topology.

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If the Flexible Algorithm definition includes “Exclude SRLG(s)”, then all links where such SRLG(s) are advertised are pruned from the topology.

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For ISIS, TI-LFA backup paths for a Flexible Algorithm are computed using the same constraints as the calculation of the primary paths. Additionally, flexible algorithm specific backup paths that are TI-LFA calculated can be SRLG disjoint.This means the primary path and backup path do not share any common SRLG(s).

Limitations

In Flexible Algorithm Specific Link Attributes (ASLA) SRLG TLV, the following are not supported:

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Advertisement is not supported for IPv6 and unnumbered ISIS interfaces.

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Decoding is not supported for IPv6 and unnumbered ISIS interfaces.