EIGRP Feasible Successor + DUAL on Cisco: Hands-On Lab

EIGRP's signature feature is sub-second convergence on common failure modes. The secret: DUAL (Diffusing Update Algorithm) precomputes loop-free backup paths called Feasible Successors (FS). When the primary path (Successor) fails, EIGRP instantly switches to the FS - no waiting for queries, no recalculation. This lab demonstrates the FS concept on the base topology and shows what happens during a topology change.

What you will learn

The terms Successor, Feasible Successor, Feasible Distance, Advertised Distance
The Feasibility Condition - the math EIGRP uses to detect loop-free backups
How to read show ip eigrp topology for FS information
What happens during DUAL recalculation when an FS exists (instant) vs does not (Active state, query/reply)

What this lab does NOT cover

Stuck-in-Active (SIA) failures - covered in the EIGRP cluster pillar
EIGRP wide-metrics

Topology

Download the EIGRP Reference Lab .yaml

Three iol-xe routers (R2, R3, R4) in a triangle of /30 point-to-point links with EIGRP named mode (PINGLABZ, AS 100) preconfigured. R2-R3: 10.30.31.0/30, R3-R4: 10.30.32.0/30, R2-R4: 10.30.33.0/30. Import, start, log in as pinglabz / PingLabz!23, observe.

Download EIGRP Reference Lab

Unlike most labs in the library, this one uses a dedicated reference topology - the base topology has only one path between any two routers, so DUAL never has a backup to compute. EIGRP is preconfigured in the .yaml (the configuration syntax is what lab ipc-10 teaches); here you import, boot, and observe the algorithm.

The four key terms

Successor

The next-hop neighbor on the best path. The route in the routing table comes from here.

Feasible Successor (FS)

A backup next-hop neighbor that EIGRP has VERIFIED is loop-free. May exist or may not.

Feasible Distance (FD)

The total metric from THIS router to the destination, through the successor.

Advertised Distance (AD)

The metric the NEIGHBOR claims to the destination - the neighbor's own FD for that prefix.

The Feasibility Condition

A neighbor X is a Feasible Successor for destination D if and only if:

AD(X, D) < FD(this router, D)

"The neighbor's advertised distance must be LESS than my current feasible distance." This guarantees the path through X cannot loop back through me (because X has a better metric to D than I do).

That inequality does more work than it looks like, and it is worth seeing it applied to a prefix that has no qualifying backup at all. The FD and AD arithmetic behind DUAL is the theory this lab is exercising, worked through on a real topology table including the case where a tie fails the condition.