EIGRP Feasibility condition
The EIGRP Feasibility condition states that:
The Advertised Distance (AD) of the Feasible Successor (FS) must be lower than the Feasible Distance (FD) of the Successor.
To understand this more fully, take a look at the following topology:
From the point of view of R2, the information found within the EIGRP topology table about the destination network is:
- FD to destination via R1: 19
- FD to destination via R3: 18
- AD of R1: 12
- AD of R3: 9
The Successor will be the route via R3 since that has the lowest FD. An FS will exist only if the feasibility condition is met. In this case the feasibility condition is:
- the AD of the FS is 12
- the FD of the successor is 18
- 12 < 18
Therefore the condition is met, and the path via R1 is installed in the topology table as a valid FS.