A Cisco IOS EIGRP feasible successor router can become successor provided the following criteria are met:
1. ADfeasible successor < FDsuccessor
2.FD feasible successor < (variance * FDsuccessor)
Here the AD is the Advertised Distance (“reported distance”) and the FD is the Feasible Distance.
*Since the default Variance = 1, EIGRP will only loadbalance between equal – cost routes once the variance is greater than one.