Hello Ahmedlmad
In the lesson, the configuration requires that all routers in the topology be configured with the RLFA parameters. This means that all routers will attempt to use RLFA. However, there are cases where some routers may use LFA in such a topology.
In situations where a suitable RLFA path is not available (due to topology constraints, lack of PQ nodes, or other reasons), Cisco routers may automatically use LFA, if an LFA path is available and feasible. This fallback is built into the OSPF implementation to ensure network resilience. According to this Cisco Documentation, RLFA and LFA work together and in conjunction with OSPF to deliver these backup reroute paths.
Topology Independent Loop Free Alternative (TI-LFA) is a more advanced form of LFA. While the standard LFA and RLFA are dependent on the topology and may not always provide a backup path, TI-LFA provides a guaranteed backup path regardless of the topology. This is achieved by using segment routing, which allows the router to pre-determine the backup path rather than relying on the network’s current state.
More info on this can be found here:
I hope this has been helpful!
Laz