EIGRP Reported and Feasible Distance

To understand how EIGRP selects the best path, walk through this topology step by step.
In this scenario, you are on R1, and you need to reach the destination network 10.0.23.0/24.

Figure 1 – EIGRP Path Selection Topology with Multiple Paths

From R1, you have multiple possible paths to reach that network.

Your task is simple: determine which path EIGRP will install in the routing table.
To do that, EIGRP must evaluate all available paths and decide which one is the best according to its metric calculation.

Successor and Feasible Successor

As you saw in previous lessons, EIGRP always selects a Successor route as the primary path.

Figure 2 – Successor Route and Feasible Successor Route in EIGRP

It also tries to maintain a Feasible Successor route as a backup path.
If the primary path fails, EIGRP can switch immediately to this backup without recalculating the entire network.

This behavior enables fast convergence.

The Role of DUAL

EIGRP makes these decisions using the DUAL (Diffusing Update Algorithm).

DUAL analyzes all learned routes stored in the topology table, not just the best one.
It selects the optimal path and determines whether a loop-free backup path exists.

The Two Key Metrics

To understand how DUAL makes its decision, you must understand two key values:

• Reported Distance (RD)

• Feasible Distance (FD)

These two values form the foundation of EIGRP path selection.
In the next section, we will define them and examine how they relate to our example topology.

The Reported Distance is the metric that a neighbor advertises to reach a destination network.

What Your Neighbor Advertises

Before making a decision, you need to know how far your neighbors are from that destination.

Figure 3 – EIGRP Topology

That information is provided through the Reported Distance (RD).

RD Propagation toward R1

Each EIGRP router advertises to its neighbors the metric it calculates locally to reach the destination.