The RFC 8518 inequalities in the preceding section for computing a backup path using an alternate ASBR or ABR node POi can in some topologies result in a path that may still traverse the best ASBR or ABR node PObest in the local area.
The SR OS implementation enhances the node S computation of the backup path by applying the following additional inequality to detect that situation:
D_opt(N,POi)+ Cost(POi,P) < D_opt(N,PObest)+ Cost(PObest,P)
Node S prefers a path using a POi node which satisfies this inequality. If there is no such POi node, and in the case of an external prefix or an anycast SID prefix of a SR-OSPF tunnel, node S attempts to compute a SR repair tunnel following the enhancement to this feature described in 7750 SR and 7950 XRS Segment Routing and PCE User Guide, section LFA solution across IGP area or instance boundary using SR repair tunnel for SR OSPF.
An example topology in which an SR repair tunnel is preferred over the overlapping IP next-hop based backup path is provided in 7750 SR and 7950 XRS Segment Routing and PCE User Guide, section Example application of MH prefix LFA with repair tunnel.
In all cases, the backup path using the POi node which does not satisfy this inequality is programmed as a last resort.