MPLS Fast Reroute is a temporary protection mechanism that allows an LSR to detect and detour traffic around a local failure. The mechanism is designed to detour traffic within 50ms, providing a path for label switched packets to use whilst a new LSP is established.
FRR is enabled at either the MPLS protocol global level or at an individual LSP level. Each ingress, transit and egress node that carries an LSP utilities detour paths for every link that the LSP traverses.
With the exception of tunnel affinity classes the detour paths carry none of the original constraints of the LSP, simply because the detour paths are only meant to be used temporarily until a secondary LSP has been established.