The local RIB may have multiple paths for a prefix. The path selection mode refers to the algorithm used to decide which paths to advertise to an add-paths peer. SR OS supports a send N path selection algorithm (See draft-ietf-idr-add-paths-guidelines) and a send multipaths selection algorithm.
The send N algorithm selects the N best advertisable paths that meet the following constraints.
The BGP next-hop of the route is unique.
The BGP route is not rejected by an export policy.
The BGP route is not blocked by a split-horizon rule.
The number of advertised paths does not exceed N. N is derived from the send-limit of the best BGP RIB-IN as applied by BGP import policy action or the configuration of the add-paths command that applies to the neighbor and the address family.
The send multipaths algorithm selects the N best advertisable paths that meet the following constraints.
The BGP route is a multipath (in other words, it is tied with the best path up to and including the NH-cost comparison step of the decision process, skipping steps that do not apply).
The BGP next-hop of the route is unique.
The BGP route is not rejected by an export policy.
The BGP route is not blocked by a split-horizon rule.