U-turn Alternates for IP/LDP Fast-Reroute
This document defines and describes the use of U-turn alternates to provide local protection for IP unicast and/or LDP traffic in the event of a single failure, whether link, node or shared risk link group (SRLG). When a topology change occurs, a router S pre-computes for each prefix an alternate next-hop that can be used if the primary next-hop fails. An acceptable alternate can be either a loop-free alternate or a U-turn alternate. A U-turn alternate uses a neighbor, whose primary next-hop to the prefix is router S itself and which has itself a loop-free node-protecting alternate, which thus does not go through router S to reach the destination prefix.