A Management Information Base (MIB) is a collection of information that is stored on the local agent of the managed device. MIBs are organized hierarchically and are accessed by the NMS. MIBs are organized in a treelike structure, with each branch containing similar objects. Each object has a unique object identifier (number) that uniquely identifies the managed object of the MIB hierarchy.

The top-level MIB object IDs belong to different standards organizations, and lower-level object IDs are allocated to associated organizations. Vendors define private branches that include managed objects for their products. Figure 16-2 shows a portion of the MIB tree structure. RFC 1213 describes the MIBs for TCP/IP. Cisco defines the MIBs under the Cisco head object. For example, a Cisco MIB can be uniquely identified by either the object name, iso.org.dod.private.enterprise.cisco, or the equivalent object descriptor,

Figure 16-2 MIB Tree Structure

Each individual manageable feature in the MIB is called a MIB variable. The MIB module is a document that describes each manageable feature that is contained in an agent. The MIB module is written in Abstract Syntax Notation 1 (ASN.1). Three ASN.1 data types are required: name, syntax, and encoding. The name serves as the object identifier. The syntax defines the object's data type (integer or string). The encoding data describes how information associated with a managed object is formatted as a series of data items for transmission on the network. More specific information about Cisco MIBs can be found at http://www.cisco.com/public/sw-center/netmgmt/ cmtk/mibs.shtml.

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