When LDP-over-RSVP is enabled, the RSVP LSP is modeled as an endpoint, that is, the destination node of the LSP, and not as a link in the IGP SPF. Thus, it is not possible for IGP to compute a primary or alternate next-hop for a prefix which FEC path is tunneled over the RSVP LSP. Only LDP is aware of the FEC tunneling but it cannot determine on its own a loop-free backup path when it resolves the FEC to an RSVP LSP.
As a result, LDP does not activate the LFA next-hop it learned from RTM for a FEC prefix when the FEC is resolved to an RSVP LSP. LDP activates the LFA next-hop as soon as the FEC is resolved to direct primary next-hop.
LDP FEC tunneled over an RSVP LSP because of enabling the LDP-over-RSVP feature therefore does not support the LDP FRR procedures and follows the slow path procedure of prior implementation.
When the user enables the lfa-only option for an RSVP LSP, as described in Loop-Free Alternate calculation in the presence of IGP shortcuts, the LSP is not used by LDP to tunnel an LDP FEC even when IGP shortcut is disabled but LDP-over-RSVP is enabled in IGP.