LDP Fast Re-route (FRR) is a technology which helps the router to reduce the MPLS traffic loss in cases of convergence during network failure. A router's convergence time is in general in the order of hundreds of milliseconds, but some applications may be very sensitive to data-loss. This technology helps the router to minimize the MPLS traffic loss by calculating and installing alternate backup paths prior to failure.

LDP FRR improves convergence in case of a single link or single node failure in the network. Convergence times will be in the order of 10s of milliseconds (Max convergence - 50 milliseconds). This is important to some application services (like VoIP) which are sensitive to traffic loss when running over the MPLS network.

Without FRR, link and/or node failures inside an MPLS LDP network result in traffic loss in the order of 100s of milliseconds. The reason for that is that LDP depends on the convergence of the underlying IGP (IS-IS sending LSPs/ OSPF sending LSAs in this case). Post IGP convergence, LDP itself needs to compute new primary Next-Hop Label Forwarding Entries (NHLFEs) for all affected Forwarding Equivalence Classes (FECs). Finally, the different Label Forwarding Information Bases (LFIBs) are updated.

When FRR is configured on a node, the node pre-computes primary NHLFEs for all FECs and in addition it will pre-compute backup NHLFEs for all FECs. The backup NHLFE corresponds to the label received for the same FEC from a Loop-Free Alternate (LFA) next-hop.