R2 is currently using R3 as the successor to 192.168.1.0/24. In addition, R2 currently lists R1 as an FS, as shown in Figure 1.
The show ip eigrp topology output for R2 in Figure 2 verifies that R3 is the successor and R1 is the FS for the 192.168.1.0/24 network. To understand how DUAL can use a FS when the path using the successor is no longer available, a link failure is simulated between R2 and R3.
Before simulating the failure, DUAL debugging must be enabled using the debug eigrp fsm command on R2, as shown in Figure 3. A link failure is simulated using the shutdown command on the Serial 0/0/1 interface on R2.
The debug output displays the activity generated by DUAL when a link goes down. R2 must inform all EIGRP neighbors of the lost link, as well as update its own routing and topology tables. This example only shows selected debug output. In particular, notice that the DUAL FSM searches for and finds an FS for the route in the EIGRP topology table.
The FS R1 now becomes the successor and is installed in the routing table as the new best path to 192.168.1.0/24, as shown in Figure 4. With an FS, this change in the routing table happens almost immediately.
As shown in Figure 5, the topology table for R2 now shows R1 as the successor and there are no new FSs. If the link between R2 and R3 is made active again, then R3 returns as the successor and R1 once again becomes the FS.