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6 message format for snmp operations, 1 format of command messages, 1 message header – Contemporary Control Systems Compact Managed Switches Software Manual for Console Access User Manual

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TD020850-0MG

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5.1.6 Message Format for SNMP Operations

Five SNMP operations are used in SNMP version 1

: get, get-next, set, get-response, trap.

The first four commands are used to send and receive information for managed objects and use

the same message format. Trap uses a different format discussed in Section 5.1.6.2.

5.1.6.1 Format of Command Messages

Each command message contains a header and a protocol data unit (PDU).

5.1.6.1.1 Message Header

The fields in the message header contain

:

Version — 0, indicating SNMP version 1.
Community string — the community string, which authorizes NMS access to the switch.

5.1.6.1.2 Message Protocol Data Unit (PDU)

SNMPv1 PDUs contain a specific command (get, set etc.) and operands that indicate the object

instances involved in the transaction.
The fields in the PDU contain

:

PDU type — indicates the command type : get(0xA0), get-next(0xA1), set(0xA3), get-response(0xA2)
Request ID a 4-octet integer used to match response to queries
Error Status
— a single octet integer containing a value of zero in a request and the following

error status in a response

noError (0):

no problem

tooBig (1):

the response to your request was too big to fit into one response.

noSuchName (2): an agent was asked to get or set an OID that it can’t find; i.e., the OID

doesn’t exist. It can be used for an unsupported object.

badValue (3):

a read-write or write-only object was set to an inconsistent value.

readOnly (4):

this error is generally not used.

genErr (5):

none of the previous errors.

Error Index — a single octet integer that associates an error with a particular object identifier

(OID). Only the response operation sets this field. Other operations set this field to zero.
Variable binding — contains a sequence of OIDs and values

Figure 55 —

Format of the Command Message for SNMP Version 1

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