EIGRP: Feasible Distance, Feasibility Condition

FD is a record of the lowest known distance since the last transition from the Active to the Passive state.  It is a historical record of the smallest known CD to a specific destination with the history resetting at each Active-Passive transition.

FD1.PNG

FD is not necessarily equal to the current and best CD to a destination.  There is exactly one FD per destination regardless of how many neighbors there are with routes to that destination.  FD is an internal variable that’s never advertised to any other router.  While the CD can change as metrics are modified/updated…the FD will not change…

FD2.PNG

FD comes into play when considering loop free paths to a specific network destination.  When a FD is set to a destination (for example 2560) when a topology change occurs the router is aware that any neighbors that provided this path must have been even closer (less than 2560).  In other words their RDs must have been less than 2560.  Any neighbor whose RD was less than 2560 was considered a safe path and could be used for redundancy routing.

This is what allows other routers participating in EIGRP to not have immediately up to date information, any neighbor who’s advertised a RD less than the FD of the router is trusted to have a path that is not looped back through that router.  This is what’s referred to as the Feasibility Check.

Neighbors who pass this check where their advertised RD is less than the FD of the router is considered to have satisfied the Feasibility Condition…

The Feasibility Condition is also referred to as the Source Node Condition

 

 

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