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 in the 7450 ESS, 7750 SR, 7950 XRS, and VSR Unicast Routing Protocols Guide, 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.