The Managed Device as a Conceptual Data Store

A MIB is best thought of as a conceptual data store. Managers can retrieve management information from the MIB by directing corresponding requests at the management agent—for example, using a "get" operation. In many cases, they can also manipulate and modify the information that is contained in the MIB—for example, using a "set," a "create," or a "delete" operation.

The MIB, of course, is not the same as a database. The MIB does not store information about the real world (the actual managed device) in a file system; instead, it is actually "connected" to the real world and simply offers a view of it. In other words, it offers an abstraction of the managed device that is used for management purposes.

When a manager retrieves a piece of information from a MIB, it represents an aspect of the device—for example, an internal register that has kept track of the number of packets that were received over a port, or a setting for a protocol timeout parameter. When a manager manipulates the information in a MIB, the actual settings of the device are modified, affecting the way that the device behaves in the real world. Management information hence provides the knobs that network managers can turn to control the device, and the displays that tell network managers everything they need to know to manage the device. MIBs are one of the central concepts in network management, and their importance cannot be overemphasized.

MIBs contain many individual pieces of management information about the managed entity— information about physical aspects such as ports and line cards, as well as about logical aspects such as protocol machines, software, and features of individual communication services. The pieces of management information in a MIB are commonly referred to as managed objects (MOs)—abstractions of individual aspects of the managed device that are not decomposed further for management purposes but are treated as one informational entity. In general, those aspects correspond to the "nouns" that are the subject of management conversations between managers and agents. Here are some examples:

■ Retrieve statistical information about a port (that is used to connect a piece of equipment to a

■ Create an access control rule (that specifies for a firewall which packets to filter)

■ Configure the connection endpoint of an ATM connection Each of the nouns in italics could be represented by its own MO.

Figure 6-1 shows a typical depiction of a MIB: a conceptual database that is associated with a management agent and that contains a number of MOs. MOs in MIBs are often shown arranged in conceptual tree structures. This is done because, in many cases, MOs have hierarchical containment relationships with each other. For example, an MO that represents an equipment chassis may contain other MOs that represent line cards, or an MO that represents a communications interface may contain other MOs that represent subinterfaces of that same interface. Similarly, in many cases, the names by which MOs are referred to are hierarchical in nature, not unlike the way in which postal addresses in the real world are hierarchical (a person's name at the number of a street of a city and zip code of a state). In the case of a MIB, MOs might have names such as "the number of a particular type of interface on a certain port on a certain line card" (as before, words printed in italics denote the different levels of the hierarchy). Details on how MOs are named depend on the metaschema—that is, the specification language that is used to model management information, a concept which is described later in this chapter, and on the management protocol that is used to access the management information. Management protocols are discussed in detail in Chapter 8, "Common Management Protocols: Languages of Management."

Figure 6-1 MIB and MOs network)

Management Agent

The actual real-world aspects of the entity being managed that MOs represent are referred to as real resources or managed resources, to distinguish them from their management abstraction, the managed objects. Just as the entirety of all real resources constitutes the managed real-world entity, the entirety of all MOs constitutes the managed device's MIB. In effect, the managed device consists of a "real resource plane" that exists independent of its need to be managed; the "management plane" provides the management infrastructure and views on it. Figure 6-2 depicts the relationship between the different terms.

Figure 6-2 MIB and MOs, Managed Entity and Real Resources represents

Managed Objects

Chassis MO

represents

' Ca

-d MOs

r1

represent

Management Plane Real Resource Plane

Managed Entity

Real Resources

Managed Entity

Real Resources

It should also be mentioned that, in addition to information about the real resources themselves, a MIB can contain information about how those resources relate, modeled as relationships between MOs. As mentioned earlier, the most common case is that of hierarchical relationships—for example, a chassis contains a card, a card contains a port, and a port contains a connection endpoint.

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