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.
Use of RSVP shortcut is supported only with LDR FRR LFA. It is not supported in the main SFP.
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 will not have an LFA next-hop. It will rely on RSVP FRR for protection.
The LFA SPF is extended to use IGP shortcuts as LFA next-hops as described in IS-IS and OSPF support for Loop-Free Alternate calculation.