Feasible Successor

bighornsheep

To the Cisco experts....I'm trying to figure out the concepts of feasible distance, feasible successors in the metric calculation of EIGRP.

I think I've figured out calculating the metric part out of the way, but not so sure about the selection of feasible successors.

Supposed I had on Router 1, I have two routes to network A, through R2, R3.
From R2, net-A has AD of 2,000,000, from R3, net-A has AD of 40,000,000
The cost to net-A thru R2 is 41,000,000, and thru R3 is 42,000,000

Assume I'm only using default values of k, so metric = b/w + delay and all lines have the same delay.

So obviously the route thru R2 has lowest metric and it enters the routing table, 41,000,000 becomes the FD. Now my question is...will be route thru R3 having AD of 40,000,000 become FS? Or will EIGRP still consider the route thru R2 has having the lower AD than FD but it is already the FD, so no FS gets selected?

mikearama

Based on your numbers, the path through R3 should be added as an FS.

The AD through R3 is 40,000,000... less than the FD through R2. So though R2 gets to be the successor, R3 should be added to the topology table as the FS.

Mike

bighornsheep

Thanks Mike, I sat down and calculated those numbers based on delay and bandwidth that I was using. I was playing around with a 6-router setup and trying to learn feasibility in the scenario..but I think it was getting too late and I must have entered something incorrectly, or forgot to clear the neighbors table..so it didn't select any feasible successor.

After reading what you said, and thinking about it, I don't see why it wouldn't select R3 as FS...I've saved my configs at home, so I'll load everything up and try it once again.

ps. I think I might have to start contributing to the "too much cisco" thread....all I can think about these days is routing...routing..routing...