Lowest-cost paths are then inserted by the DUAL algorithm into the routing table. These primary routes are known as successor routes. A copy of the successor paths is placed in the topology table.

EIGRP keeps important route and topology information readily available in a neighbor table and a topology table. These tables supply DUAL with comprehensive route information in case of network disruption. DUAL selects alternate routes quickly by using the information in these tables.

If a link goes down, DUAL looks for an alternative route path, or feasible successor, in the topology table. If a feasible successor is not found, the route is flagged as active, or unusable at present. Query packets are sent to neighboring routers requesting topology information. DUAL uses this information to recalculate successor and feasible successor routes to the destination.

Once DUAL has completed these calculations, the successor route is placed in the routing table. Then both the successor route and feasible successor route are placed in the topology table. The route to the final destination will now pass from an active status to a passive status. This means that the route is now operational and reliable.

EIGRP routers establish and maintain adjacencies with neighbor routers by using small hello packets. Hellos are sent by default every five seconds. An EIGRP router assumes that, as long as it is receiving hello packets from known neighbors, those neighbors and their routes remain viable, or passive.

When newly discovered neighbors are learned, the address and interface of the neighbor is recorded. This information is stored in the neighbor data structure. When a neighbor sends a hello packet, it advertises a hold time. The hold time is the amount of time a router treats a neighbor as reachable and operational.In other words, if a hello packet is not heard from within the hold time, the hold time expires. When the hold time expires, DUAL is informed of the topology change, and must recalculate the new topology.

In the example in Figures - , DUAL must reconstruct the topology following the discovery of a broken link between router D and router B.

The new successor routes will be placed in the updated routing table.