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3.1.1. Prefix-SID Algorithm

Segment Routing supports the use of multiple routing algorithms, i.e, different constraint-based shortest-path calculations can be supported. An algorithm identifier is included as part of a Prefix-SID advertisement. Specification of how an algorithm-specific path calculation is done is required in the document defining the algorithm.

This document defines two algorithms:

  • Shortest Path First: this algorithm is the default behavior. The packet is forwarded along the well known ECMP-aware Shortest Path First (SPF) algorithm employed by the IGPs. However, it is explicitly allowed for a midpoint to implement another forwarding based on local policy. The Shortest Path First algorithm is, in fact, the default and current behavior of most of the networks where local policies may override the SPF decision.

  • Strict Shortest Path First (Strict-SPF): This algorithm mandates that the packet be forwarded according to the ECMP-aware SPF algorithm and instructs any router in the path to ignore any possible local policy overriding the SPF decision. The SID advertised with the Strict-SPF algorithm ensures that the path the packet is going to take is the expected, and not altered, SPF path. Note that Fast Reroute (FRR) [RFC5714] mechanisms are still compliant with the Strict Shortest Path First algorithm. In other words, a packet received with a Strict-SPF SID may be rerouted through an FRR mechanism. Strict-SPF uses the same topology used by the Shortest Path First algorithm. Obviously, nodes that do not support Strict-SPF will not install forwarding entries for this algorithm. Restricting the topology only to those nodes that support this algorithm will not produce the desired forwarding paths since the desired behavior is to follow the path calculated by the Shortest Path First algorithm. Therefore, a source SR node MUST NOT use an SR Policy containing a strict SPF segment if the path crosses a node not supporting the Strict-SPF algorithm.

An IGP-Prefix segment identifies the path, to the related prefix, computed as per the associated algorithm. A packet injected anywhere within the SR domain with an active Prefix-SID is expected to be forwarded along a path computed using the specified algorithm. For this to be possible, a fully connected topology of routers supporting the specified algorithm is required.

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