V6100 and V7122 User Guide 357The ‘tree’ structure described above is an integral part of the SNMP standard, though themost pertinent parts of the tree are the ‘leaf’ objects of the tree that provide actualmanagement data regarding the device. Generally, SNMP leaf objects can be partitioned intotwo similar but slightly different types that reflect the organization of the tree structure: Discrete MIB Objects - Contain one precise piece of management data. These objectsare often distinguished from ‘Table’ items (below) by adding a ‘.0’ (dot-zero) extension totheir names. The operator must merely know the name of the object and no otherinformation. Table MIB Objects - Contain multiple sections of management data. These objects aredistinguished from ‘Discrete’ items (above) by requiring a ‘.’ (dot) extension to theirnames that uniquely distinguishes the particular value being referenced. The ‘.’ (dot)extension is the ‘instance’ number of an SNMP object. For ‘Discrete’ objects, thisinstance number is zero. For ‘Table’ objects, this instance number is the index into theSNMP table. SNMP tables are special types of SNMP objects which allow parallel arraysof information to be supported. Tables are distinguished from scalar objects, so thattables can grow without bounds. For example, SNMP defines the ‘ifDescr’ object (as astandard SNMP object) that indicates the text description of each interface supported bya particular device. Since network devices can be configured with more than oneinterface, this object can only be represented as an array.By convention, SNMP objects are always grouped in an ‘Entry’ directory, within an objectwith a ‘Table’ suffix. (The ‘ifDescr’ object described above resides in the ‘ifEntry’ directorycontained in the ‘ifTable’ directory).SNMP Extensibility FeatureOne of the principal components of an SNMP manager is a MIB Compiler which allows newMIB objects to be added to the management system. When a MIB is compiled into an SNMPmanager, the manager is made ‘aware’ of new objects that are supported by agents on thenetwork. The concept is similar to adding a new schema to a database.Typically, when a MIB is compiled into the system, the manager creates new folders ordirectories that correspond to the objects. These folders or directories can typically beviewed with a MIB Browser, which is a traditional SNMP management tool incorporated intovirtually all Network Management Systems.The act of compiling the MIB allows the manager to know about the special objectssupported by the agent and access these objects as part of the standard object set.Carrier Grade Alarm SystemThe basic alarm system has been extended to a carrier-grade alarm system. A carrier-gradealarm system provides a reliable alarm reporting mechanism that takes into account EMSoutages, network outages, and transport mechanism such as SNMP over UDP.
