User`s manual

H.323 User's Manual 15. SNMP-Based Management
Version 5.0 345 December 2006
15 SNMP-Based Management
Simple Network Management Protocol (SNMP) is a standards-based network control
protocol for managing elements in a network. The SNMP Manager (usually implemented
by a Network Management System (NMS) or an Element Management System (EMS))
connects to an SNMP Agent (embedded on a remote Network Element (NE)) to perform
network element Operation, Administration and Maintenance (OAM).
Both the SNMP Manager and the NE refer to the same database to retrieve information or
configure parameters. This database is referred to as the Management Information Base
(MIB), and is a set of statistical and control values. Apart from the standard MIBs
documented in IETF RFCs, SNMP additionally enables the use of private MIBs, containing
a non-standard information set (specific functionality provided by the NE).
Directives, issued by the SNMP Manager to an SNMP Agent, consist of the identifiers of
SNMP variables (referred to as MIB object identifiers or MIB variables) along with
instructions to either get the value for that identifier, or set the identifier to a new value
(configuration). The SNMP Agent can also send unsolicited events towards the EMS,
called SNMP traps.
The definitions of MIB variables supported by a particular agent are incorporated in
descriptor files, written in Abstract Syntax Notation (ASN.1) format, made available to EMS
client programs so that they can become aware of MIB variables and their use.
The device contains an embedded SNMP Agent supporting both general network MIBs
(such as the IP MIB), VoP-specific MIBs (such as RTP) and AudioCodes' proprietary MIBs
(acBoard, acGateway, acAlarm and other MIBs), enabling a deeper probe into the inter-
working of the device. All supported MIB files are supplied to customers as part of the
release.
15.1 SNMP Standards and Objects
15.1.1 SNMP Message Standard
Four types of SNMP messages are defined:
 Get: A request that returns the value of a named object.
 Get-Next: A request that returns the next name (and value) of the ‘next’ object
supported by a network device given a valid SNMP name.
 Set: A request that sets a named object to a specific value.
 Trap: A message generated asynchronously by network devices. It is an unsolicited
message from an agent to the manager.
Each of these message types fulfills a particular requirement of network managers:
 Get request: Specific values can be fetched via the ‘get’ request to determine the
performance and state of the device. Typically, many different values and parameters
can be determined via SNMP without the overhead associated with logging into the
device, or establishing a TCP connection with the device.
 Get-Next request: Enables the SNMP standard network managers to ‘walk’ through
all SNMP values of a device (via the ‘get-next’ request) to determine all names and
values that an operant device supports.
 Get-Bulk: Extends the functionality of Get-Next by allowing multiple values to be
returned for selected items in the request. This is accomplished by beginning with the
first SNMP object to be fetched, fetching the next name with a "get-next", and
repeating this operation.