3.2 EIGRP Configuration  
  3.2.6 Select routes  
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.

The sophisticated algorithm of DUAL results in EIGRP having exceptionally fast convergence. To better understand convergence using DUAL, consider the example in Figure . All routers have built a topology table that contains information about how to route to destination network Z.

Each table identifies the following:

  • The routing protocol or EIGRP
  • The lowest cost of the route or Feasible Distance (FD)
  • The cost of the route as advertised by the neighboring router or Reported Distance (RD)

The Topology heading identifies the preferred primary route, which is called the successor route (Successor). If it is identified, the Topology heading will also identify the backup route, which is called the feasible successor (FS). Note that it is not necessary to have an identified feasible successor.