Diffusing update algorithm

The diffusing update algorithm, usually called DUAL, is the routing algorithm inside Cisco's EIGRP protocol. Its job is to make sure a route is recalculated across the whole network whenever a change might otherwise cause a routing loop. J.J. Garcia-Luna-Aceves developed it at SRI International. EIGRP applies a conservative feasibility condition, so only routes that cannot loop are selected: a neighbour's reported distance must be smaller than the feasible distance of the current best route. When no feasible route remains, DUAL starts a diffusing computation that clears every trace of the bad route from the network, and the Bellman-Ford algorithm then recovers a new one. It keeps neighbour, topology and routing tables for this work.