Today something that surprised me. Let’s say we have an R4 with three paths to a given destination 10.10.10.0/24:
- through R1 FD 10000, RD 2000, current successor
- through R2 FD 20000, RD 5000, current feasible successor
- through R3 FD 15000, RD 12000 !!!shorter than path through F2, but doesn’t meet the feasibility condition
My understanding of EIGRP was that if a path through R1 goes down, the feasible successor will be made the successor. It is NOT the case. What happens is as follows:
- R1 goes down
- R4 thinks: my path through R3 is now the shortest: let’s see if it meets the feasibility condition. It does not, so it puts the route in active state and sends a query to R3.
- R3 replies.
- R1 thinks now i have two paths: the shorter path is now through R3.
- R1 makes the path through R3 to be the successor. Path through R2 is still the feasible successor.