The IP Fast-Reroute (IP FRR) feature supports the use of the Loop-Free Alternate (LFA) backup next-hop to forward packets of IP prefixes when the primary next hop is not available. This allows a node to resume forwarding IP packets to a destination prefix without waiting for the routing convergence.
When any of the following events occurs, IGP instructs in the fast path the IOM or the forwarding engine to enable the LFA backup next hop.
OSPF/IS-IS interface goes operationally down: physical or local admin shutdown.
Timeout of a BFD session to a next hop when BFD is enabled on the OSPF/IS-IS interface.
IP FRR is supported on IPv4 and IPv6 OSPF/IS-IS prefixes forwarded in the base router instance to a network IP interface or to an IES SAP interface or spoke interface. It is also supported for VPRN VPN-IPv4 OSPF prefixes and VPN-IPv6 OSPF prefixes forwarded to a VPRN SAP interface or spoke interface.
IP FRR also provides a LFA backup next hop for the destination prefix of a GRE tunnel used in an SDP or in VPRN auto-bind.
The LFA next hop precomputation by IGP is described in RFC 5286 Basic Specification for IP Fast Reroute: Loop-Free Alternates.