Hi Life,
The DUAL finite-state machine takes the decision process for all route computations by tracking all routes advertised by all neighbors.
DUAL uses distance information to select efficient, loop-free paths and selects routes for insertion in a routing table based on feasible successors.
A feasible successor is a neighboring router used for packet forwarding that is a least-cost path to a destination that is guaranteed not to be part of a routing loop. When a neighbor changes a metric, or when a topology change occurs, DUAL tests for feasible successors. If one is found, DUAL uses it to avoid recomputing the route unnecessarily. When no feasible successors exist but neighbors still advertise the destination, a recomputation (also known as a diffusing computation) must occur to determine a new successor.
Although recomputation is not processor-intensive, it does affect convergence time, so it is advantageous to avoid unnecessary recomputations.
HTH
Ankur
