PLR : Point of Local Repair : The node that is just upstream of the protected resource. This node notices that the protected resource(link/node/set-of-links) is broken first and uses an FRR mechanism to protect that resource

Repair Node : The node to which the PLR wants to get the traffic to during the convergence period, once the protected resource fails, because the repair node is guanteed to have a path to the destination 'D'

Pre-Convergence State : A source 'S' is said to be in a pre-convergence state regarding a particular destination 'D' from the time the primary-path to 'D' fails until 'S' installs a new route to 'D'

Post-Convergence State : A source 'S' is said to be in a post-convergence state regarding a particular destination 'D' after it installs a new route to 'D'

P-space: The P-space of a router with respect to a protected link is the set of routers reachable from that specific router using the pre-convergence shortest paths without any of those paths (including equal-cost path splits) transiting that protected link.For example, the P-space of S with respect to link S-E is the set of routers that S can reach without using the protected link S-E

Extended P-space: Consider the set of neighbors of a router protecting a link. Exclude from that set of routers the router reachable over the protected link. The extended P-space of the protecting router with respect to the protected link is the union of the P-spaces of the neighbors in that set of neighbors with respect to the protected link

Q-space: The Q-space of a router with respect to a protected link is the set of routers from which that specific router can be reached without any path (including equal-cost path splits) transiting that protected link

PQ node: A PQ node of a node S with respect to a protected link S-E is a node that is a member of both the P-space (or the extended P-space) of S with respect to that protected link S-E and the Q-space of E with respect to that protected link S-E.

LFA :