Notes on Feasibility Condition in EIGRP

While going back over my notes I saw that I had provided minimal information about Administrative Distance, Feasible Distance, and Feasibility Condition.  So here is a post to clarify a couple of items about this, particularly the Feasibility Condition.

As a reminder:

Advertised Distance: this is also called the Reported Distance, and this is important to know!  The concept of the AD/RD is that this is the metric as seen by the neighbor router (the router advertising the EIGRP route to you).  The Advertised Distance being sent to you is basically the neighbor routers Feasible Distance.  With each “hop” this number should goes up.  Your router will calculate a Feasible Distance, and then advertise that downstream as the Advertised Distance (or Reported Distance).  This concept is key in understanding how metrics are calculated.  Each router has to determine the cost on its own.  It uses information it knows about neighbors such as the speed of the wire used to get to the neighbor, combined with the Advertised Distance to calculate its own Feasible Distance.

Feasible Distance: this is the metric as calculated by your router.  This is ultimately used to choose the best path.

Now, it should be obvious that if a router receives an EIGRP advertisement on (2) of its interfaces for the same network that the route with the lowest Feasible Distance will become the “successor” route placed in the route table.  But, this is only if the Feasible Distance is greater than the advertised distance being given by the neighbor on that interface.  Otherwise, we might have a loop.  Further more, the second route being advertisded for this subnet (presumably on the other interface) should be placed in the topology table as the “feasible successor” route.  Do you ever wonder why they call this the “feasible” successor?  Well, this route is a valid backup route for the successor only because it meets the “Feasibility Condition”, therefore it’s the feasible successor.  Plainly, this means that the Advertised Distance being given on this 2nd interface is lower than the Feasible Distance (metric) on the successor route’s interface.  So what?  Well, this means that the backup route placed in the topology table is least likely to have a loop.  At least, this is how I understand it.  Remember, feasible successor routes are not just failover routes, but they are also used to load balance….so even the feasible successor must have an Advertised Distance smaller than the Feasible Distance.

Feasiblity Condition: if a non-successor route’s Advertised Distance is less than the Feasible Distance then the Feasibility Condition has been met.

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