Cisco SRST User Guide - Page 42

SNMP Overview Chapter 1 Cisco SRST SNMP MIB Support MIB Overview In a managed device, specialized low-impact software modules, called agents access information about the device and make it available to the network management system (NMS). Managed devices maintain values for a number of variables and report those, as required, to the NMS. For example, an agent can report data such as the number of bytes and packets in and out of the device, or the number of broadcast messages sent and received. In the Internet network management framework, each variable, which is a managed object, is any information that an agent can access and report back to the NMS. All managed objects are contained in the MIB database. The managed objects can be set or read to provide information on network devices and interfaces. An NMS can control a managed device by sending a message to an agent of that managed device requiring the device to change the value of one or more of its managed objects. MIB Source MIBs come from various sources: • Standard-On the Internet Engineering Task Force (IETF) standards track at Proposed, Draft, or full standard. A Proposed Standard can change somewhat due to implementation experience. A Draft Standard changes somewhat less, with more attention to backward compatibility. A full Internet Standard doesn't change much. At all levels these are published as Requests for Comment (RFCs). • Internet Draft-IETF work in progress. Sometimes the best way to instrument technology is with an Internet Draft MIB, which is typically being worked on by an IETF working group. Such MIBs are somewhat unstable, so it is necessary to capture the specific Internet Draft and to place the MIB within the Cisco Enterprise MIB space (not in the Experimental branch). • Cisco-Cisco enterprise-specific (also called proprietary or private, even though publicly documented). Such MIBs add instrumentation not covered by standard MIBs. As of Cisco IOS Release 10.2, Cisco has old MIBs and new MIBs. The old MIBs are from older software versions and often have somewhat unconventional features. • Other companies-Non-Cisco enterprise-specific. It is occasionally appropriate to implement a MIB defined by some other company, especially for technology they originated and instrumented. This presents problems like these associated with Internet Drafts in that a version of the MIB definition must be captured, but the MIB itself should remain wherever in the MIB space the originating company put it so as to easily support existing applications. MIB Objects A MIB is conceptually a tree (as shown in Figure 1-4), where the leaves are the individual data objects. An object can be, for example, a counter or a protocol status. The SNMP framework uses the term "object" in a way different from the way OSI management uses it. An OSI object is a network entity, such as a router or a protocol, which has attributes. These OSI attributes and SNMP objects are essentially the same concept, that is, individual data values. A MIB object consists of the following values: • Object type-Identifies the type of MIB object. • Syntax-Identifies the data type which models the object. • Access-Identifies the maximum level of access and can have one of five values (listed from highest to lowest level of access): - Read-create-Indicates that instances of the object may be read, written, and created 1-28 Cisco SRST SNMP MIB Release 3.4 Guide OL-7959-01