3Com VCX V7122 User Manual

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3Com VCX V7122 User Manual Manual pdf 174 page image

3Com VCX V7122 SIP VoIP Gateway User Manual174can be determined via SNMP without the overhead associated with logging into thedevice, or establishing a Transmission Control Protocol (TCP) connection with thedevice. Get Next Request - Enables the SNMP standard network managers to ‘walk’ through allSNMP values of a device (via the ‘get-next’ request) to determine all names and valuesthat an operant device supports. This is accomplished by beginning with the first SNMPobject to be fetched, fetching the next name with a ‘get-next’, and repeating thisoperation. Set Request - The SNMP standard provides a method of effecting an action associatedwith a device (via the ‘set’ request) to accomplish activities such as disabling interfaces,disconnecting users, clearing registers, etc. This provides a way of configuring andcontrolling network devices via SNMP. Trap Message - The SNMP standard furnishes a mechanism by which devices can‘reach out’ to a Network Manager on their own (via a ‘trap’ message) to notify or alert themanager of a problem with the device. This typically requires each device on the networkto be configured to issue SNMP traps to one or more network devices that are awaitingthese traps.The above message types are all encoded into messages referred to as Protocol Data Units(PDUs) that are interchanged between SNMP devices.SNMP MIB ObjectsThe SNMP MIB is arranged in a tree-structured fashion, similar in many ways to a diskdirectory structure of files. The top level SNMP branch begins with the ISO ‘internet’directory, which contains four main branches: The ‘mgmt’ SNMP branch - Contains the standard SNMP objects usually supported (atleast in part) by all network devices. The ‘private’ SNMP branch - Contains those ‘extended’ SNMP objects defined bynetwork equipment vendors. The ‘experimental’ and ‘directory’ SNMP branches - Also defined within the ‘internet’ rootdirectory, these branches are usually devoid of any meaningful data or objects.The ‘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 by