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EIGRP DUAL algorithm — explain Successor and Feasible Successor.

Successor — the route with the lowest metric to a destination, installed in the routing table. Feasible Successor (FS) — a backup route that satisfies the Feasibility Condition: FS reported distance < Successor feasible distance. FS is pre-computed, kept in topology table, and installed instantly if Successor fails — no recomputation needed. This is what gives EIGRP sub-second convergence. If no FS exists, DUAL goes Active and queries neighbors — slower.
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