DUAL Finite-State Machine

When EIGRP loses its successor or primary route, EIGRP immediately tries to reconverge by looking at its topology table to see if any feasible successors are available. If a feasible successor is available, EIGRP immediately promotes the feasible successor to a successor and informs its neighbors about the change. The feasible successor then becomes the next hop for EIGRP to forward the packets to the destination. The process by which EIGRP converges locally and does not involve other routers in the convergence process is called local computation. This also saves CPU power because all the feasible successors are already chosen before the primary route failures. (Refer to Figure 6-3.) If the primary route (Router D) is not available for some reason, the preselected feasible successor Router H immediately takes over as the primary route.

Now, if the primary route goes away and no feasible successors are available, the router goes into diffused computation. In diffused computation, the router sends query packets to all its neighbors asking for the lost route, and the router goes into Active state. If neighboring routers have information about the lost route, they reply to the querying router. If neighboring routers do not have information about the lost route, they send queries to all their neighbors. If the neighboring router does not have an alternate route and doesn't have any other neighbors, it sends a reply packet back to the router with a metric set to infinity, indicating that it, too, doesn't have an alternate route available. The querying router waits for all the replies from all its neighbors and then chooses the neighbor with the best metric in its replies as the next hop to forward packets.

Referring to Figure 6-3, if the primary successor Router B is not available and its feasible successor Router H is also not available, Router A sends a query to Router D asking for Network 7. In this case, Router D simply replies to the query with a valid metric to Network 7. Router A then converges using Router D as its next hop to Network 7.

To sum up the operation of DUAL, DUAL selects a successor as the primary path and also selects a feasible successor as its backup path based on the feasibility condition. If the successor becomes unavailable, the feasible successor is used as the primary route. If the feasible successor is not present, the router queries all its neighbors and computes a new successor based on the replies to the queries. Therefore, in an EIGRP network, the query mechanism is the only means to achieve fast convergence.

Chapter 8 of the Cisco Press book Routing TCP/IP, Volume 1, by Jeff Doyle, provides an excellent , detailed description of the operation of the EIGRP DUAL algorithm.