TCP/IP Structure of Management Information (SMI) and Management Information Bases (MIBs) Overview
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Defining MIB Objects: the Structure of Management Information (SMI)
The use of MIB objects solves the problem of the network management protocol being tied to the network management information. However, we must be very particular about how we define these objects. Again, the reason is the wide variety of devices that TCP/IP allows to be connected together. Each device may represent information in a different way; for them all to communicate with each other, we need to ensure that management information is represented in a consistent manner.
The part of the SNMP Framework that ensures the universality of MIB objects is the Structure of Management Information (SMI) standard. SMI defines the rules for how MIB objects and MIB modules are constructed. In SMI, MIB objects are described using a precise set of definitions based on a data description language called the ISO Abstract Syntax Notation 1 (ASN.1) standard.
In essence then, we really have three levels of abstraction in SNMP. The actual SNMP protocol moves values that represent the state of management devices. The MIB defines what these variables are. And the SMI defines how the variables in the MIB are themselves defined.
There are two main SMI standards. The original, SMIv1, was part of the first SNMP Framework, SNMPv1, defined in RFC 1155. It sets out the basic rules for management information bases and MIB variables. The second, SMIv2, was defined as part of SNMPv2p in RFC 1442 and further updated in RFC 2578, part of SNMPv3. It is similar to the earlier version but defines more object types as well as the structure of MIB modules.
These SMI standards are responsible for defining the following important information elements in SNMP:
In fact, the SMI and MIB standards are so closely related that it is hard to discuss one without the other. The rest of this section discusses SNMP management information structures making reference to both SNMP Framework components. We'll see how MIB objects are defined and the characteristics of each, and look at MIB data types, the structure used to organize MIB objects, and the way MIB modules and object groups work.
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