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Feasibility Condition and Path Selection with EIGRP

My friend, extremely skilled CCIE and collegue sent me this diagram showing a topoligy the other night. We were discussing metric weights and path selection with EIGRP.

The task was this “Calculate which routes different routers in this diagram will install to their routing tables. For instance if R2 is configured with a variance of 2, will it install the route it gets from R4?”

First i started to calculate. The shortest path based on metrics depicted is the direct route via R1. The Feasible Distance would be 200.

Alternate Routes and FD’s

R2 -> R4 -> R1 FD: 400
R2 -> R3 -> R4 -> R1 FD: 400

With a variance of 2, and looking at other metrics:
FD 200 * 2 = 400. So my first thought was “yes it would install all routes..”
But hey, whats that for loop-prevention was my next thought. If we send traffic to R2, its possible that its route to the network is via ourselves?!

Yes, this is where Feasibility Check comes in.

The rule of feasibility condition states that any route has to report a lower Reported Distance than the current Feasible Distance in order to pass the feasibility check and be able to become a feasible successor.
What does that mean? Lets start with those terms, RD and FD.

RD – Reported Distance
FD – Feasible Distance

The Reported Distance, is simply the distance a router reports to another one. Lets say the route to vlan x in the picture above is advertised with eigrp from R1 to R2, it has to Report its distance to the network.

R1 Reports its distance of 100 to R2. Because R1 has a distance of 100 to the destination.

R2 then adds its own distance to R1 to the path, in other words you can say that when the route arrives to R2, it includes only the distance from R1. Therefore R2 has to add its distance to R1 so the metric now includes the full path from R2 -> R1 – > destination.
In R2’s perspective the RD is the metric reported from R1. The FD is the RD plus the metric to reach R1 in order to summarize the paths distance.

Back to the task

The question was if the router R2 would install a route coming from R3 or R4 with RD of 400, when its FD to the destination is 200. The variance configured states that the metric difference is accepted to loadbalance, but the rule of feasibility condition that we just went through says otherwise.

Since the Reported Distance in both of this alternate paths is  higher that the current FD of R2, the paths would not be considered to be installed in the routing table of R2. The Feasibility Condition makes sure that the alternate path always is loop-free. This rule makes sure that alternate paths never depends on a route back via the router itself.

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