CCIE EIGRP Topology

[chinpohpang851]

I'm studying the CCIE course now and came across this Feasible Distance. In the book it mentioned FD is the record of the lowest known distance since last transition from Active to Passive state. So, a series of changes in delay of the links causes CD to change hence lowest value is taken and remain as FD.

But in later example, a link down event causing the FD to reset to a higher value than previously known value, why? 

What event will trigger FD to reset and abandon the known lowest value?

[Richard Burts]

We do not have enough information to be able to address the details of your question. But we can answer these parts in general terms:

- "What event will trigger FD to reset and abandon the known lowest value?" The event that will trigger FD to reset and adopt a new value is a transition from active to passive.

- "a link down event causing the FD to reset to a higher value than previously known value, why" the logical explanation is that the link down event caused EIGRP to transition the prefix to active, to send queries, and to identify a new path which would produce a new FD.

[Martin L]

going active from passive state so that EIGRP DUAL algorithm runs and calculate new FD.

we had some discussion on this , see https://community.cisco.com/t5/routing/eigrp-why-is-the-fd-a-historical-record-for-fastethernet-but-not/m-p/4088936#M334580

Regards, ML
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