Diffusing Update Algorithm
A convergence algorithm based on research conducted at SRI Int'l that is used in Enhanced IGRP. It provides loop-free operation at every instant throughout a route computation and allows simultaneous route synchronization with a topology change. DUAL selects routes for the table based on feasible successors, i.e., a neighbor router with a least-cost path that is not part of a loop. If there are no successors but only neighbors advertising the destination, the route is recomputing a new successor is selected. DUAL will test for feasible successors with a topology change; if there are none, DUAL will use any it finds in order to avoid recomputation. Note that the recomputation is not processor intensive, but it does affect convergence time.
DUAL is based calculations that were first proposed by E.W. Dijkstra and C.S. Scholten and heavily researched by J.J. Garcia-Luna-Acaves (at SRI).
Use the show ip eigrp topology command to determine DUAL states and debug DUAL problems.