How does SNMP gets its information to answer the management station requests ? What is in fact a MIB ? I keep reading about the MIB but nobody says what a MIB is in its practical aspect.
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MIB = Management Information Base
A MIB is in essence a schema document describing the hierarchical structure of an SNMP tree. It provides meaning to arbitrary numbers. It turns a returned value of "0.1812414156" into "CPU Load = 18%".
The SNMP daemon has methods built into it for turning requested OIDs and returning values. This varies for every single OID and SNMP daemon, so there is no one answer. In some cases a script is fired off to collect the needed information, in others there are procedures baked into SNMP daemon itself that collect the needed information.
MIBs can be provided with different pieces of software and are used by management/monitoring software in order for them to understand what the monitored software is doing. As an example you may install a VMware product, which has MIBs, and then you may use WhatsUpGold to monitor the VMware environment. MIBs describe the alerts/language of the thing you want to monitor to your monitoring software.