How does the DUAL algorithm in EIGRP ensure loop-free routes?

• A. By maintaining feasible successors that satisfy the feasibility condition to prevent loops.
• B. By using flood-fill of link-state information to all routers.
• C. By restricting updates to only one route per destination.
• D. By always choosing the path with the lowest metric as the sole path.

Answer: (A)

The key idea is the feasibility condition that DUAL uses to identify safe backup paths. Each router keeps the best path to a destination (the successor) and may also hold a feasible successor as a backup. A candidate backup path is considered feasible only if the distance reported by the neighbor for that destination is strictly less than the router’s current feasible distance to that destination. This constraint guarantees that the neighbor’s path to the destination won’t route traffic back through the advertising router, which would create a loop. Because the neighbor’s distance is proven to be closer, traffic can move toward the destination without circling back, making the route loop-free. If the primary path fails, the router can quickly switch to a feasible successor without risking a loop, and even share traffic across multiple safe paths when appropriate.

Other descriptions don’t capture this mechanism: flooding full link-state information is a feature of other protocols, not how EIGRP’s DUAL prevents loops; limiting updates to a single route per destination isn’t how EIGRP operates; and always choosing the lowest metric as the sole path ignores the available feasible successors that provide safe, potentially unequal-cost load balancing.