EIGRP Path Selection

In the previous lesson, you calculated two total metrics from R1 toward 10.0.23.0/24: 3072 via R3 and 3328 via R2.

At this point, you have two possible paths to reach 10.0.23.0/24.

R1 compares all available paths.
The path via R3 has the lowest total metric (3072).

That value becomes R1’s Feasible Distance (FD), and this path is selected as the Successor.

Defining the Successor Route

The Successor is simply the path with the lowest Feasible Distance.

Figure 1 – Successor Route with Feasible Distance and Reported Distance

When a route becomes the Successor, EIGRP:

• Installs it in the routing table

• Marks it as the successor in the topology table

• Displays both its FD and RD

Here is what you see on R1:

R1# show ip eigrp topology
EIGRP-IPv4 Topology Table for AS(1)/ID(1.1.1.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

P 10.0.23.0/24, 1 successors, FD is 3072
        via 10.0.13.2 (3072/2816), GigabitEthernet0/1 <--- Successor Route
        via 10.0.12.2 (3328/2816), GigabitEthernet0/2

For the network 10.0.23.0/24, you can see two paths.

The first entry is the Successor Route because it has:

• The lowest total metric (FD = 3072)

• A Reported Distance of 2816

As you move forward, remember this:
EIGRP, through the DUAL algorithm, always tries to keep a backup path ready.