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Routing & Switching · Intermediate

Diffusing Update Algorithm (DUAL)

The EIGRP algorithm that guarantees loop-free paths and selects successors and feasible successors.

Definition

DUAL tracks each neighbor's reported distance and the router's own feasible distance per prefix. A neighbor is a loop-free backup if its reported distance is less than the current feasible distance. If the successor fails and a feasible successor exists, DUAL switches instantly; otherwise the route goes active and queries are sent.

Example

show ip eigrp topology shows routes in Passive (P) or Active (A) state.

Exam tip

Passive means stable; Active means DUAL is querying. 'Active' is the bad state, a frequent trick.

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