The SNMP agent: what the manager asks, what the device answers and what it says on its own
Every mechanism has a step-by-step animation, the configuration explained line by line and exam notes. Wherever a parameter changes the outcome, you get a simulator where you change it yourself.
TL;DR
The SNMP agent is the process on the device that exposes its state as a virtual database (the MIB), in which every value has a numeric address (an OID). The manager asks on UDP 161 — Get, GetNext, GetBulk, Set — and receives a Response; the agent speaks on its own only through traps and informs, on UDP 162, and only if you told it what to report and to whom. v1 and v2c authenticate with a clear-text community, v3 with users that use HMAC and AES; whatever the version, VACM decides which subtree each one sees. Most incidents come from the community ACL, from v3 users lost together with the engineID, from 32-bit counters that wrap and from traps sent in the wrong VRF.
How to read the notes: red = critical for the exam, green = theory, blue = numbers, yellow = pitfalls, cyan = differences between platforms and standards. In the animations, the configuration lines used in the current step are highlighted automatically; the ← → arrows change the step when the animation is selected.
The analogy, in short
Think of a building manager with the meter ledger: the utility company sends a meter reader who asks “what does page 2 say, on the row for apartment 2?”, and the manager answers from the ledger. He does nothing unasked — except leaving a message on the company’s answering machine when the boiler stops.
The model holds up to §10. From §11 onward (RMON, EEM, telemetry), the manager does things the SNMP protocol does not define; see the full analogy and its limits in §15.