IP SLA: the router that tests its own paths
Every mechanism has a step-by-step animation, its 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
IP SLA is active monitoring: the router itself sends test traffic (probes) towards a destination and measures whether it answers, how fast and with what loss. An operation has a type (icmp-echo, udp-jitter, tcp-connect, http, dns…), timers (frequency, timeout, threshold) and a schedule. Its result becomes a return code, which a track object turns into Up / Down: static routes, PBR, HSRP and EEM react to the track, not to IP SLA. For jitter, per-direction loss and voice quality, the destination must be a Cisco responder. Reactions send traps or syslog when thresholds are crossed. Most incidents come from probes not anchored on the tested path, unscheduled operations and timers in the wrong units.
How to read the notes: red = exam-critical, green = theory, blue = numbers, yellow = pitfalls, cyan = differences between platforms and standards. In the animations, the configuration lines used at the current step are highlighted automatically; the ← → arrows change the step when the animation is focused.
The analogy, in brief
Think of the dispatcher of a bus line who cannot see the street: at a fixed interval they send a test run to the end of the line, time it, and switch on a red light on the map when the runs no longer arrive — at which point the buses with passengers are moved onto the detour route.
The model holds until §10. From §11 onwards (scaling, LSP ping, Y.1731), it breaks down; see the full analogy and its limits in §15.