DUAL (Diffusing Update Algorithm) is how EIGRP calculates paths and handles failures.
EIGRP maintains distances for each route:
Feasible Distance (FD): The best metric from this router to the destination.
Reported Distance (RD): The metric a neighbor advertises. Also called Advertised Distance.
DUAL selects paths using the feasibility condition:
A neighbor's RD must be less than the current FD.
This guarantees the neighbor has a loop-free path. If the primary path fails and a feasible successor exists, EIGRP switches instantly. Only if no feasible successor exists does EIGRP query neighbors.