EIGRP can store a backup route in its topology table, incase of the primary route being removed from the routing table.
The primary route of EIGRP is known as the successor route, and the backup route the feasable successor.
One condition needs to be met for a route to become a feasible successor:
The Reported Distance (RD) of the feasible successor needs to be lower than the Feasible Distance (FD) of the successor in the topology table.
R1# sh ip eigrp topology
IP-EIGRP Topology Table for AS(2014)/ID(18.104.22.168)
Codes: P – Passive, A – Active, U – Update, Q – Query, R – Reply,
r – reply Status, s – sia Status
P 22.214.171.124/24, 1 successors, FD is 409600
via 192.168.13.3 (409600/128256), FastEthernet0/1 ————–> Successor route FD
via 192.168.12.2 (412160/156160), FastEthernet0/0 —————-> Feasible successor RD
This is just something that needs to be memorized.