OK I'll try. Here we go : (Sorry it became a bit longer than first expected)
We assume that we're looking at router A. And let's say router A has a feasible distance = 100 (the cost the router is using to get to some dest.). Furthermore router A is using router B as next-hop (router B has let's say an AD of 50 and the cost to get to B from A is 50 (so FD = 100).
Now A has another neighbor C which reports an AD of 150 (A's cost of reaching C could be 10, so we would have a total cost of 160 to get to the dest. via C --- But ----
Router C advertises AD=150. This is higher than router A's FD=100. The problem is that router A has no way of knowing whether this AD (from C) is calculated as router A's AD plus one or more link costs which gives the total of 150. Router A therefore can't rule out that he's included in router C's path to get to the dest..
On the other hand if router C's AD is lower than router A's own FD, then router A can be sure that he is not in the path router C is using, And router C therefore is a Feasible Successor.
If we change the AD value from C - from 150 to 95 - then we would have a distance of 105 through router C. And still a distance of 100 through router B.
But now the AD=95 from C is lower than the used FD=100. So we know that router A wouldn't make a loop if it went through router C.
Regards
Mikael (Going to CCIE Lab on tuesday the 20.)
