The LOC-RIB may have multiple paths for a prefix. The path selection mode refers to the algorithm used to decide which of these paths to advertise to an add-paths peer. SR OS supports the Add-N path selection algorithm described in draft-ietf-idr-add-paths-guidelines. The Add-N algorithm selects, as candidates for advertisement, the N best paths with unique BGP next-hops. In the SR OS implementation, the default value of N is configurable, per address-family, at the BGP instance, group and neighbor levels, however, this default value can be overridden, for specific prefixes, using route policies. The maximum number of paths to advertise for a prefix to an add-paths neighbor is the value N assigned by a BGP import policy to the best path for P, otherwise it defaults to the neighbor, group or instance level configuration of N for the address family to which P belongs.
Add-paths allows non-best paths to be advertised to a peer, but it still complies with basic BGP advertisement rules such as the iBGP split horizon rule: a route learned from an iBGP neighbor cannot be readvertised to another iBGP neighbor unless the router is configured as a route reflector.