When BGP fast reroute is enabled the IOM reroutes traffic onto a backup path based on input from BGP. When BGP decides that a primary path is no longer usable it notifies the IOM and affected traffic is immediately switched to the backup path.
The following events trigger failure notifications to the IOM and reroute of traffic to backup paths.
The peer IP address is unreachable and the peer-tracking is enabled.
The BFD session associated with BGP peer goes down.
The BGP session terminated with peer (for example, send/receive NOTIFICATION).
There is no longer any route (allowed by the next-hop resolution policy, if configured) that can resolve the BGP next-hop address.
The LDP tunnel that resolves the next-hop goes down. This could happen because there is no longer any IP route that can resolve the FEC, or the LDP session goes down, or the LDP peer withdraws its label mapping.
The RSVP tunnel that resolves the next-hop goes down. This could happen because a ResvTear message is received, or the RESV state times out, or the outgoing interface fails and is not protected by FRR or a secondary path.
The BGP tunnel that resolves the next-hop goes down. This could happen because the BGP label-IPv4 route is withdrawn by the peer or else becomes invalid because of an unresolved next-hop.