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 topologyfor 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.
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
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).
