EIGRP - FD is not the same of any metrics

rachelhor · ‎06-06-2008

hi,

I was doing some lab tests, and manipulated the bandwidth and delay so that I manage to get a feasible successor.

However, what puzzles me why is the feasible distance of a particular route is not the same as any metrics of the 2 possible paths (via main and via feasible successor)?

I read that the FD (feasible distance) of a route should be the same as the best metric of a path.

Can someone explain to me why I get an FD that is not the same of any metrics?

Also, I read in an article that says "Even with variance configured, EIGRP will not send traffic over an unequal cost path if the reported distance is greater than the feasible distance for that particular route".

In view that a router is a feasible successor, the reported distance must be less than the feasible distance.

In other words, does it mean that we cannot have unequal load balancing if there's no feasible successor?

Tks!

Rachel

bhatok · ‎06-06-2008

Hi Rachel,

Can you post a show ip route and a show ip eigrp topology?

FD is the metric to the remote network.

From my lab:

show ip route

D 10.254.254.12 [90/3072] via 10.254.254.18, 00:06:48, Serial1/1

[90/3840] via 10.254.254.5, 00:06:48, Serial1/0

show ip eigrp top

P 10.254.254.12/30, 1 successors, FD is 3072

via 10.254.254.18 (3072/768), Serial1/1

via 10.254.254.5 (3840/1536), Serial1/0

The FD is 3072 and 3840 from show ip eigrp top, in the show ip route output you can also see these two numbers after the administrative distance of eigrp, which is 90.

From what I understand (and my lab testing) you are correct that you cannot have unequal cost load balancing without a feasible successor.

Brandon

ruwhite · ‎06-09-2008

If EIGRP has a route with a successor and a feasible successor, and it switches from the successor to the feasible, it will not run DUAL, it does not send queries, so the FD will not be updated, we just switch to the new route. In this case, the FD may not match the metric of the route we are using to reach the destination. I'm guessing this is the case here, since you're saying you've switched things around a good bit in your lab.

Try clearing the route out of the topo table, and see if that helps--it's a hidden command, clear ip eigrp topo , I think is the complete command. That simply forces the route active, which should, I think, cause the router to recalculate the FD.

:-)

Russ

rachelhor · ‎06-10-2008

hi Russ,

From your explanation, though I don't fully understand, it is likely to be my case.

What is the difference between "successor" and "feasible successor"? I thought they are the same. :P

tks!

rachel

royalblues · ‎06-10-2008

Successor is the best route to the destination and is placed in to the routing table

Feasible successor on the other hand is the second best route to the destination and you can see them in the topology table

Narayan