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A diffusing update algorithm (DUAL or DUAL finite state machine) is a convergence algorithm that dictates a routing protocol used by Cisco’s proprietary Enhanced Interior Gateway Routing Protocol (EIGRP) to prevent routing loops via a continuous route computation.
A DUAL finite state machine (FSM) consists of many different states and scenarios. The DUAL protocol scans all routes to track the optimal path, in terms of efficiency and cost. Then, it adds that path in the routing table. DUAL FSM also manages backup routes, in case the primary and most efficient route is lost.
This term is also known as the DUAL finite state machine (DUAL FSM).
Routing protocols use several techniques to prevent any loop, which is an unreachable destination causing data packets to bounce back. Loops must be prevented because they hamper the performance of the entire network. Looped packets may have to be retransmitted to ensure that transmission is not due to overflow or other delivery failure.
UDP loops can be persistent, as there is no feedback on receipt. So, a sending application may keep sending packets into a network black hole. These receive data at full speed of the line, causing network performance degradation. Distance vector routing protocols, such as EIGRP, use several techniques to avoid looping, including the diffusing update algorithm.
DUAL avoids looping and uses an algorithmic calculation to ascertain best and alternate paths.
DUAL maintains several metrics about the paths, including the successor, feasible distance, feasible successor, reported distance and feasible condition.