CCNP ROUTE: 2. EIGRP Successor and Feasible Successor

Studying the EIGRP Successor and Feasible Successor concepts (on R6):

R6#sho ip eigrp topology
IP-EIGRP Topology Table for AS(1)/ID(172.16.111.111)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

P 192.168.44.0/24, 1 successors, FD is 2297856
        via 172.16.0.2 (2297856/128256), Serial0/0
P 192.168.55.0/24, 1 successors, FD is 409600
        via 172.16.0.6 (409600/128256), FastEthernet0/1
P 172.16.0.12/30, 1 successors, FD is 2195456
        via 172.16.0.6 (2195456/2169856), FastEthernet0/1
        via 172.16.0.2 (2681856/2169856), Serial0/0
P 172.16.0.8/30, 1 successors, FD is 2169856
        via Connected, Serial0/1
P 172.16.0.4/30, 1 successors, FD is 281600
        via Connected, FastEthernet0/1
P 172.16.0.0/30, 1 successors, FD is 2169856
        via Connected, Serial0/0
P 172.16.111.0/24, 1 successors, FD is 128256
        via Connected, Loopback111

Feasible Distance (FD): Integer metric for the route, from the local router’s pespective, used by the local router to choose the best route for that prefix. It is the distance from the local router to the destination network.
Reported Distance (RD): Integer metric for the route, from the neighboring router’s perspective (the neighbor that told the local router about the route). It is the distance from the neighboring router to the destination network, advertised to the local router. (aka AD - Advertised Distance)

-R6 has 2 possible ways to get to the network between R4 and R5 (172.16.0.12/30), as seen above.
-R6 installs both routes in the topology table, but only the Successor (172.16.0.6) in the routing table, for its best metric (FD): 2195456<2681856.
-The Feasible Successor is there as a backup in case the Successor fails and it will be installed in the routing table instantly after the Successor route fails, without any additional calculations.
-The FD to the destination network is 2195456. The AD from the neighbor to the destination is 2169856.

Feasibility condition: If a non-successor route’s RD is less than the FD, the route is a feasible successor route.

In this case, 2169856 is less than 2195456, so 172.16.0.2 is considered as a Feasible Successor.

As a result, only 172.16.0.6 will be installed in the routing table as a next-hop for 172.16.0.12/30:
R6#show ip route
...
D    192.168.44.0/24 [90/2297856] via 172.16.0.2, 00:24:24, Serial0/0
     172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks
D       172.16.0.12/30 [90/2195456] via 172.16.0.6, 00:24:24, FastEthernet0/1
C       172.16.0.8/30 is directly connected, Serial0/1
C       172.16.0.4/30 is directly connected, FastEthernet0/1
C       172.16.0.0/30 is directly connected, Serial0/0
C       172.16.111.0/24 is directly connected, Loopback111
D    192.168.55.0/24 [90/409600] via 172.16.0.6, 00:24:24, FastEthernet0/1