In SR OS, fast reroute is optional and must be enabled by using either the BGP backup-path command or the route-policy install-backup-path command. Typically, only one approach is used.
The backup-path command in the base router context is used to control fast reroute on a per-RIB basis (IPv4, label-IPv4, IPv6, and label-IPv6). When the command specifies a particular family, BGP attempts to find a backup path for every prefix learned by the associated BGP RIB.
The install-backup-path command, available in route-policy-action contexts, marks a BGP route as requesting a backup path. It only takes effect in BGP import and VRF import policies. If only some prefixes should have backup paths, then the backup-path command should not be used, and instead the install-backup-path command should be used to mark only those prefixes that require extra protection.
In general, a prefix supports ECMP paths or a backup path, but not both. The backup path is the best path after the primary path and any paths with the same BGP next-hop as the primary path have been removed.