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EIGRP: Why is the FD a historical record for Fastethernet, but not Gigabitethernet?

dk3874
Level 1
Level 1

Regarding the post "Definition Of Feasible Distance In EIGRP Convergence":

https://community.cisco.com/t5/routing/definition-of-feasible-distance-in-eigrp-convergence/m-p/4087265#M334424

Peter Paluch's (Hall of Fame Cisco Employee Hall of Fame Cisco Employee) description of the FD:

"Feasible Distance is the lowest distance to the destination experienced since the last time the route went from Active to Passive state. Put differently, the Feasible Distance is the historical record of how closest the router was to the destination since the last diffusing computation for the destination has finished."

My question: Is there a purpose as to why the FD is a historical record, rather than the FD changing higher when the CD (Calculated Distance) changes higher (and the destination prefix does not go from Active to Passive)?

Is this just an inadvertent quirk as to how Cisco implemented EIGRP or is this by design for some reason?

I started testing in GNS3 what Peter was describing and inadvertently I found another reason to ask this question.

I noticed that with my Fastethernet interfaces, the output matched exactly what Peter was describing, in that unless the destination prefix went from Active to Passive, the FD did not change when the CD would go higher, however, I found that the FD does automatically go higher when using Gigabitethernet interfaces. I am not sure if this is unique to GNS3, whether there is a quirk that was fixed with EIGRP as it relates to Gig interfaces or some other reason.

If I have all FE's interfaces connected and I raise the delay along the path and then I do a show ip eigrp topology all-links, I do see the CD change higher, but the FD does not move.

If I have all GE's interfaces connected and I raise the delay along the path and then I do a show ip eigrp topology all-links, I see both the CD and FD go higher (i.e. their metric is both higher and match each other).

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So actually I have two questions:

Is there a purpose for the FD to act as a historical record? i.e Why does the FD not move higher (when the CD moves higher) without the destination prefix having to go from Active to Passive? 

And why do I only see the FD act as a historical record with FastEthernet interfaces, but not with GigabitEthernet interfaces?

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