Diffusing Update Algorithm (DUAL)

EIGRP uses Diffusing Update Algorithm (DUAL) for selecting and maintaining the best route to each remote network. DUAL is also used for the following tasks:

• discover a backup route if there’s one available.
• support for variable length subnet masks (VLSMs).
• perform dynamic route recoveries.
• query neighbors for unknown alternate routes.
• send out queries for an alternate route.

EIGRP stores all routes advertised by all neighbors. The metric of these routes is used by DUAL to select efficient loop free paths. DUAL selects routes that will be inserted into a routing table. If a route fails, and there is no feasible successor, DUAL chooses a replacement route, which usually takes a second or two.
The following requirements must be met for DUAL to work properly:

• EIGRP neighbors must discovered.
• all transmitted EIGRP messages should be received correctly.
• all changes and messages should be processed in the order in which they’re detected.