EIGRP: backup routes computed before the failure
Every mechanism has a step-by-step animation, the configuration explained line by line and exam notes. Where a parameter changes the outcome, you get a simulator where you change it yourself.
TL;DR
EIGRP is an advanced distance vector protocol: each router learns routes only from its neighbors, through Hellos and Updates sent reliably (IP protocol 88). The default metric combines the minimum bandwidth on the path with the cumulative delay. DUAL picks the successor and, when possible, keeps a feasible successor ready — guaranteed loop-free — that takes over instantly on failure. Without a backup, the router asks its neighbors (Query) and waits for all the replies; stubs and summarization limit how far the questions travel. On top of this core you add unequal-cost load balancing, filtering, redistribution, authentication, named mode, IPv6 and integration with provider networks.
How to read the notes: red = critical for the exam, green = theory, blue = numbers, yellow = pitfalls, cyan = RFC ↔ Cisco differences. In the animations, the configuration lines used at the current step are highlighted automatically; the ← → arrows change the step when the animation has focus.
The analogy, in short
Think of mountain huts connected by radio stations. Each hut (router) learns only from its neighbors how far each peak (prefix) is, follows the best guide (successor) and, when possible, keeps a backup one ready, guaranteed safe (feasible successor).
The model holds up to §12. From §13 onward (IPv6, PE-CE, OTP), it breaks down; see the full analogy and its limits in §18.