When an RSVP LSP is used as a shortcut by IGP, it is included by SPF as a P2P link and can also be optionally advertised into the rest of the network by IGP. Thus the SPF is able of using a tunneled next-hop as the primary next-hop for a specific prefix. LDP is also able of resolving a FEC to a tunneled next-hop when the IGP shortcut feature is enabled.
When both IGP shortcut and LFA are enabled in IS-IS or OSPF, and LDP FRR is also enabled, then the following additional LDP FRR capabilities are supported:
A FEC which is resolved to a direct primary next-hop can be backed up by a LFA tunneled next-hop.
A FEC which is resolved to a tunneled primary next-hop does not have an LFA next-hop. It relies on RSVP FRR for protection.
The LFA SPF is extended to use IGP shortcuts as LFA next-hops as described in Loop-Free Alternate calculation in the presence of IGP shortcuts.