Diffusing update algorithm

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DUAL stands for Diffusing Update ALgorithm and is the algorithm used by EIGRP to calculate and create routing tables based on certain criteria.
DUAL is used to ensure a path to a network and provide a loopless routing environment. In order to help ensure delivery of a packet to a network, DUAL sends out query packets to its adjacent neighbors, or directly connected routers. As query packets are sent out, each router continues to forward the query packet until a router responds with a reply packet that has information on how to reach the specific network. When the reply packets are received by the router that sent out the query packets, DUAL calculates which route will be the successor and feasible successor.

The successor is the main route. It is the shortest, most efficient route to a network that DUAL can find. The route to the successor router is calculated using hop count, bandwidth, and other factors. The feasible successor is the backup route. The router that is the feasible successor has the second-best route to the network. It is not necessary for EIGRP to have a feasible successor.

When the successor or feasible successor goes down, DUAL sends out query packets to each router, and places the route in its routing table as "active." When the route is active, DUAL is recalculating the path to the new network. Once it is calculated, the route is marked as "passive," and normal network operation can resume.