30.5 Examples of MIB Variables
Versions 1 and 2 of SNMP each collected variables together in a single large MIB, with the entire set documented in a single RFC. After publication of the second generation, MIB-II, the IETF took a different approach by allowing the publication of many individual MIB documents that each specify the variables for a specific type of device. As a result, more than 100separate MIBs have been defined as part of the standards process; they specify more than 10,000individual variables. For example, separate RFCs now exist that specify the MIB variables associated with devices such as: a hardware bridge, an uninterruptible power supply, an ATM switch, and a dialup modem. In addition, many vendors have defined MIB variables for their specific hardware or software products.
Examining a few of the MIB data items associated with TCP/IP protocols will help clarify the contents. Figure 30.3 lists example MIB variables along with their categories.
Most of the items listed in Figure 30.3 are numeric -- each value can be stored in a single integer. However, the MIB also defines more complex structures. For exarnple, the MIB variable ipRoutingTable refers to an entire routing table. Additional MIB variables define the contents of a routing table entry, and allow the network management protocols to reference an individual entry in the table, including the prefix, address mask, and next hop fields. Of course, MIB variables present only a logical definition of each data item -- the internal data structures a router uses may differ from the MIB definition. When a query arrives, software in the agent on the router is responsible for mapping between the MIB variable and the data structure the router uses to store the information.
Abstract from Internetworking With TCP/IP Vol I: Principles, Protocols, and Architecture Fourth Edition,
DOUGLAS E. COMER,
Department of Computer Sciences Purdue University, West Lafayette, IN 47907,
PRENTICE HALL,
Upper Saddle River, New Jersey 07458