Mib

KEY POINT

A MIB is a collection of managed objects. A MIB stores information, which is collected by the local management agent, on a managed device for later retrieval by a network management protocol.

Each object in a MIB has a unique identifier that network management applications use to identify and retrieve the value of the specific object. The MIB has a tree-like structure in which similar objects are grouped under the same branch of the MIB tree. For example, different interface counters are grouped under the MIB tree's interfaces branch.

Internet MIB Hierarchy

As shown in Figure 3-28, the MIB structure is logically represented by a tree hierarchy. The root of the tree is unnamed and splits into three main branches: Consultative Committee for International Telegraph and Telephone (CCITT), ISO, and joint ISO/CCITT.

These branches and those that fall below each category are identified with short text strings and integers. Text strings describe object names, whereas integers form object identifiers that allow software to create compact, encoded representations of the names. The object identifier in the Internet MIB hierarchy is the sequence of numeric labels on the nodes along a path from the root to the object. The Internet standard MIB is represented by the object identifier 1.3.6.1.2.1, which can also be expressed as iso.org.dod.internet.mgmt.mib.

Figure 3-28 Internet MIB Hierarchy

This information was adapted from the Cisco Management Information Base (MIB) User Quick Reference, which is available at http://www.cisco.com/univercd/cc/td/doc/product/software/ ios112/mbook/index.htm.

Standard MIBs are defined in various RFCs. For example, RFC 1213, Management Information Base for Network Management of TCP/IP-based internets: MIB-II, defines the TCP/IP MIB.

In addition to standard MIBs, vendors can obtain their own branch of the MIB subtree and create custom managed objects under that branch. A Cisco router MIB uses both standard and private managed objects.

A Cisco router's MIB tree contains several defined standard managed objects, including from the following groups:

■ Interface group (including interface description, type, physical address, counts of incoming and outgoing packets, and so forth)

■ IP group (including whether the device is acting as an IP gateway, the number of input packets, the number of packets discarded because of error, and so forth)

■ ICMP group (including the number of ICMP messages received, the number of messages with errors, and so forth)

The Cisco private section of the MIB tree contains private managed objects, which were introduced by Cisco, such as the following objects for routers:

■ Small, medium, large, and huge buffers

■ Primary and secondary memory

■ Proprietary protocols

Private definitions of managed objects must be compiled into the NMS before they can be used; the result is output that is more descriptive, with variables and events that can be referred to by name.

MIB-II

MIB-II is an extension of the original MIB (which is now called MIB-I) and is defined by RFC 1213. MIB-II supports a number of new protocols and provides more detailed, structured information. It remains compatible with the previous version, which is why MIB-II retains the same object identifier as MIB-I (1.3.6.1.2.1).

The location of MIB-II objects is under the iso.org.dod.internet.mgmt subtree, where the top-level MIB objects are defined as follows (definitions of these objects can be found in RFC 1213):

■ Address Translation (3)

ICMP (5)

TCP (6)

UDP (7)

EGP (S)

Transmission (10)

SNMP (11)

Although the MIB-II definition is an improvement over MIB-I, the following unresolved issues exist:

■ MIB-II is still a device-centric solution, meaning that its focus is on individual devices, not the entire network or data flows.

■ MIB-II is poll-based, meaning that data is stored in managed devices and a management system must request (poll) it via the management protocol; the data is not sent automatically.

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