The centerpiece of the new EIGRP
technologies is DUAL, the EIGRP route-calculation engine. The full
name of the EIGRP engine is DUAL finite-state machine. This engine
contains all the logic used to calculate and compare routes in an
EIGRP network. DUAL tracks all the routes advertised by neighbors
and uses the composite metric of each route to compare them.
Selected paths must be loop-free and have the lowest cost.
Such routes are inserted by the DUAL protocol into a routing table
for use in forwarding datagrams.
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Routes selected for insertion in a routing table
are also evaluated on the basis of feasible succession. A feasible
successor is a neighbor router that is the next hop in a
least-cost path to any given destination. A feasible successor is a
path that is loop free according to the DUAL FSM.
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