3Com VCX V7122, Руководство пользователя

Страница: 364 / 488

364
V7122 GatewayUser Guide
ƒ 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:
ƒ
Get request: Specific values can be fetched using the ‘get’ request to determine the
performance and state of the device. Typically, many different values and parameters
can be determined using SNMP without the overhead associated with logging into the
device, or establishing a TCP connection with the device.
ƒ Get Next request: Enables the SNMP standard network managers to ‘walk’ through all
SNMP values of a device (using the ‘get-next’ request) to determine all names and
values that an operant device supports. 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.
ƒ
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.
ƒ Set Request: The SNMP standard provides a method of effecting an action associated
with a device (using the ‘set’ request) to accomplish activities such as disabling
interfaces, disconnecting users, clearing registers, and the like. This provides a way of
configuring and controlling network devices using SNMP.
ƒ Trap message: The SNMP standard furnishes a mechanism by which devices can
‘reach out’ to a Network Manager on their own (using a ‘trap’ message) to notify or alert
the manager of a problem with the device. This typically requires each device on the
network to be configured to issue SNMP traps to one or more network devices that are
awaiting these 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 Objects
The SNMP MIB is arranged in a tree-structured fashion, similar in many ways to a disk
directory structure of files. The top level SNMP branch begins with the ISO ‘internet’
directory, which contains four main branches:
ƒ "mgmt " SNMP branch: Contains the standard SNMP objects usually supported (at least
in part) by all network devices.
ƒ "private " SNMP branch: Contains those ‘extended’ SNMP objects defined by network
equipment vendors.
ƒ
"experimental" and "directory" SNMP branches: Also defined within the ‘internet’ root
directory, these branches are usually devoid of any meaningful data or objects.