Each agent consists of a collection of managed objects that explain the capabilities and behavior of the agent in an abstract form. This is no different from the method by which a DeviceNet device is described by a collection of objects. The objects supported by a DeviceNet limit switch differ from that of a DeviceNet pneumatic manifold block; however, all DeviceNet devices support some common objects. This is the same situation with agents. All SNMP agents must support a common set of managed objects, called a Management Information Base (MIB). But an agent must support, at a minimum, what is defined in RFC 1213: MIB-2.

You might ask what happened to MIB-1? In the ever-changing Internet world, MIB-2 superseded MIB-1. Before we examine the details of MIB-2, we need to understand the structure and naming convention of MIBs. The Structure of Management Information (SMI) is described in RFC 1155. First, we will study the naming convention for managed objects and the MIBs themselves, which are simply a collection of managed objects. The term to identify an object is simply the Object ID (OID).

OBJECT ID
Managed objects within an agent are organized into a tree-like hierarchy similar to the way files and folders are used to represent the contents of a hard disk. In fact, some NMS software displays the management objects in a graphical fashion as if they were indeed files. However, the nomenclature is different. Managed objects are identified as a series of integers separated by dots representing the nodes on a tree. Naming begins at the root, followed by branches and ending in leaves. Let me give an example. In Figure 3 you will see the tree structure for finding MIB-2. It begins at the root on the left. There are three branches, but we are interested only in iso(1). From iso(1) we have three more branches, but we are only interested in org(3). Next there are six more branches, but we follow dod(6). From this branch we go to internet(1). At this node we are at the base of all SNMP MIBs. The short form for representing where we are is 1.3.6.1 or we could say iso.org.dod.internet.

Figure 3 — The identification of objects follows a hierarchical structure.

At this point we could follow either mgmt(2) or private(4) branches. If we follow the mgmt(2) branch, we will find standard MIBs. If we follow the private(4) branch, we will find vendor-specific MIBs. This is where a vendor can register unique products with corresponding unique management information. For example, a UPS would have much different information to share than an Ethernet switch. We will follow the mgmt branch and locate MIB-2 which is at 1.3.6.1.2.1 or you could simply say mgmt(1) which uniquely identifies its location.

We have found MIB-2, but we do not know the location of the individual managed objects. It’s best to remember that MIB-2 is a collection of objects and each object description is identified in RFC 1213. If we study RFC 1213, we will learn there are ten managed object groups in MIB-2.

The first object group is system. The system group lets you enter the physical location of the device, the name of the device and who is responsible for the device. Therefore, if the device is queried by a management system, it could say it was tagged UPS-1, located in the pump house and if there is trouble to call Randy in the Instrument Shop. Another attribute of this object is up-time. It will continue to accumulate time until it is unpowered.

The 10 Managed Object Groups in MIB-2
Group	OID	Comment
system	{ mib-2 1 }	General information about device for administrative purposes
interfaces	{ mib-2 2 }	Keeps track of each interface on device
at	{ mib-2 3 }	Address translation (only for backward compatibility)
ip	{ mib-2 4 }	Tracks IP (internet Protocol) aspects
icmp	{ mib-2 5 }	Tracks ICMP (Internet Control Message Protocol) aspects
tcp	{ mib-2 6 }	Tracks TCP (Transmission Control Protocol) aspects
udp	{ mib-2 7 }	Tracks UDP (User Datagram Protocol) aspects
egp	{ mib-2 8 }	Tracks EGP (Exterior Gateway Protocol) aspects
———	{ mib-2 9 }	(no longer used)
transmission	{ mib-2 10 }	Currently not used
snmp	{ mib-2 11 }	Tracks SNMP (Simple Network Management Protocol) aspects

Table 1 — The 10 Managed Object Groups in MIB-2