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1.4. The Structure of Management Information and MIBS

The Structure of Management Information (SMI) provides a way to define managed objects and their behavior. An agent has in its possession a list of the objects that it tracks. One such object is the operational status of a router interface (for example, up, down, or testing). This list collectively defines the information the NMS can use to determine the overall health of the device on which the agent resides.

The Management Information Base (MIB) can be thought of as a database of managed objects that the agent tracks. Any sort of status or statistical information that can be accessed by the NMS is defined in a MIB. The SMI provides a way to define managed objects, while the MIB is the definition (using the SMI syntax) of the objects themselves. Like a dictionary, which shows how to spell a word and then gives its meaning or definition, a MIB defines a textual name for a managed object and explains its meaning. Chapter 2, "A Closer Look at SNMP" goes into more technical detail about MIBs and the SMI.

An agent may implement many MIBs, but all agents implement a particular MIB called MIB-II [3] (RFC 1213). This standard defines variables for things such as interface statistics (interface speeds, MTU, octets[4] sent, octets received, etc.) as well as various other things pertaining to the system itself (system location, system contact, etc.). The main goal of MIB-II is to provide general TCP/IP management information. It doesn't cover every possible item a vendor may want to manage within its particular device.

[3]MIB-I is the original version of this MIB, but it is no longer referred to since MIB-II enhances it.

[4]An octet is an 8-bit quantity, which is the fundamental unit of transfer in TCP/IP networks.

What other kinds of information might be useful to collect? First, there are many draft and proposed standards developed to help manage things such as frame relay, ATM, FDDI, and services (mail, DNS, etc.). A sampling of these MIBs and their RFC numbers includes:

But that's far from the entire story, which is why vendors, and individuals, are allowed to define MIB variables for their own use.[5] For example, consider a vendor that is bringing a new router to market. The agent built into the router will respond to NMS requests (or send traps to the NMS) for the variables defined by the MIB-II standard; it probably also implements MIBs for the interface types it provides (e.g., RFC 2515 for ATM and RFC 2115 for Frame Relay). In addition, the router may have some significant new features that are worth monitoring but are not covered by any standard MIB. So, the vendor defines its own MIB (sometimes referred to as a proprietary MIB) that implements managed objects for the status and statistical information of their new router.

[5]This topic is discussed further in the next chapter.

TIP: Simply loading a new MIB into your NMS does not necessarily allow you to retrieve the data/values/objects, etc. defined within that MIB. You need to load only those MIBs supported by the agents from which you're requesting queries (e.g., snmpget, snmpwalk). Feel free to load additional MIBs for future device support, but don't panic when your device doesn't answer (and possibly returns errors for) these unsupported MIBs.

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