BGP, from the first session to EVPN
Every mechanism has a step-by-step animation, the configuration explained line by line and the exam notes. Where a parameter changes the outcome, you get a simulator in which you change it yourself.
TL;DR
BGP is the protocol through which autonomous systems (ASes) tell each other which prefixes they can reach and through which chain of ASes. Neighbors are configured manually, the session runs over TCP 179 and carries UPDATEs with attributes. Each router picks a single path using a fixed list of criteria and advertises it onward, passed through policies. Inside the AS, iBGP has rules of its own (split horizon, unchanged next hop), solved by route reflectors and by the IGP. Through MP-BGP, the same machinery carries IPv6, MPLS VPNs and MAC addresses for VXLAN. Security comes from layers added on top: GTSM, max-prefix, filters and RPKI.
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 while the animation is selected.
The analogy, in short
Think of courier companies that cooperate. Each company (AS) tells its partners which areas it can deliver to (prefixes) and through which companies the parcel would pass (AS_PATH). The depots (routers) pick the best offer using fixed rules, and the company decides through policies who it works with.
The model holds up to §10. From §11 onward (recursive next hop, MPLS labels, EVPN) it breaks down; see the full analogy and its limits in §19.