EIGRP routers keep route and topology information available in RAM, so changes can be reacted to quickly. Like OSPF, EIGRP keeps this information in several tables or databases.

The EIGRP distance vector algorithm, DUAL, uses the information gathered in the neighbor and topology tables and calculates the lowest cost route to the destination. The primary route is called the successor route. When calculated, DUAL places the successor route in the routing table and a copy in the topology table.

DUAL also attempts to calculate a backup route in case the successor route fails. This is called the feasible successor route. When calculated, DUAL places the feasible route in the topology table. This route can be called upon if the successor route to a destination becomes unreachable or unreliable.