Feasible distance

[gornication]

Hi!

  Feasible distance - the metric value for the lowest-metric path to reach a destination.(official cert guide)
 

  So, the metric value for the lowest-metric. It seems that only successor route has the lowest metric. But in the same book you can find "successor route FD" and "feasible successor FD". So the question.

Can FD be an attribute of a route that is not a successor route?

[gornication]

I meant that FD and the best metric of the best route are not the same in your reasoning and I am trying to find a way to agree with this.

[Richard Burts]

You say "FD and the best metric of the best route are not the same in your reasoning". Yes I would say that they are not the same. FD is a historical record of the metric for a prefix when EIGRP transitioned from active to passive for that prefix. FD is not used to make routing decisions. FD is used only to evaluate whether there might be a feasible successor and whether some route identified by variance can be added to the routing table. 

The value of the metric fo the best route can change and routing decisions will be made using the new value. The value of FD does not change until EIGRP transitions that prefix to the active state.

I hope that these explanations can help you agree that FD and the metric of the best route are not the same.

[gornication]

You have an interesting vision of the protocol, but contradicting what I read in other sources.

[gornication]

"No", - is FD an attribute of SR only?

"each prefix in passive state has its own FD ", - FD can be an attribute of not only SR but also other routes?

[Richard Burts]

FD is the historical record of the metric when a destination prefix changed from active to passive. FD is not a part of any particular successor route or any feasible successor route or of any other route to that destination.

[gornication]

Thanks for your answers, Richard.

To simplify, for me it sounds like "the metric is a record of the route metric, not the route metric".
It seems to me that any metric is a historical record of the moment when this metric was defined for the route. The metric that can be seen after the show route is equal to the FD value in the output of the show eigrp topology command. 

[Richard Burts · Accepted solution]

For each prefix in the routing table it is possible that there could be multiple paths to reach that prefix. Each of those paths would have its own metric. For each prefix in the routing table there is a single Feasible Distance. The FD is an attribute of the destination prefix and not an attribute of any particular path to that destination. The metric for a particular destination prefix might change at some point in time. If the metric for the destination changed without requiring a transition to the active state then the metric changes but the FD remains the same.

You say " The metric that can be seen after the show route is equal to the FD value in the output of the show eigrp topology command." This is frequently true but not always true. Here is a simple example. If you were to have a network that included subnet 10.10.10.0/24 we might find entries for it in the eigrp topology table. Perhaps the first entry has a metric of 100 and a second entry has a metric of 110. So the entry with metric of 100 becomes the successor route and the entry with metric of 110 becomes the feasible successor route. The entry in the IP routing table shows and entry with metric of 100 and a FD of 100. Then something happens to the successor route and it is withdrawn from the routing table. Because there is a feasible successor there is no need to transition to the active state. The routing table is updated with the new route whose metric is 110. What is the FD now? The FD continues to be 100 while the route metric is 110.

[gornication]

That is, for the subnet 10.10.10.0/24 FD is 2816 (because the smallest metric of the best route is 2816)and there are routes to 10.10.10.0/24:
10.10.10.0/24 -> 192.168.1.1 with metric 2816
10.10.10.0/24 -> 172.16.0.1 with metric 30720
So we have a route 10.10.10.0/24 -> 192.168.1.1 with metric 2816 and FD 2816.
And if 192.168.1.1 dies, and the RD of the route through 172.16.0.1 is less than the FD, we will get a passive working route
10.10.10.0/24 -> 172.6.0.1 with metric 30720 and FD 2816, which reflects the history of what happened, but serves not for this, but to check for compliance with the feasible condition.

Right?

[Richard Burts]

Yes that is correct. We have been discussing the main use of FD which is to determine whether an alternate route qualifies as a Feasible Successor route. Let me also point out another use of FD which is in configuration of variance in EIGRP. With variance EIGRP can add an inferior route to the active IP routing table as long as the metric of the inferior route is within some multiple of the metric of the selected best route. But just because the inferior route metric is within that range does not automatically place the route into the routing table. The inferior route must also meet the feasibility condition (which uses FD) to be inserted into the routing table.

[gornication]

Richard, thank you very much.

[Richard Burts]

You are welcome.