The centerpiece of EIGRP is DUAL and its EIGRP route-calculation engine. The actual name of this technology is DUAL Finite State Machine (FSM). This FSM contains all of the logic used to calculate and compare routes in an EIGRP network. The figure shows a simplified version of the DUAL FSM.

An FSM is an abstract machine, not a mechanical device with moving parts. FSMs define a set of possible states that something can go through, what events cause those states, and what events result from those states. Designers use FSMs to describe how a device, computer program, or routing algorithm reacts to a set of input events.

FSMs are beyond the scope of this course. However, the concept is used to examine some of the output from EIGRP’s FSM using the debug eigrp fsm command. Use this command to examine what DUAL does when a route is removed from the routing table.