Simple Network Management Protocol (SNMP) …

SNMP Reference: SNMPv2 Message Format

SNMPv2 messages consist of a header and a PDU. Figure 56-7 illustrates the basic format of an SNMPv2 message.

Figure 56-7: SNMPv2 Messages Also Consist of a Header and a PDU

SNMPv2 Message Header

SNMPv2 message headers contain two fields: Version Number and Community Name.
The following descriptions summarize these fields:

• Version number—Specifies the version of SNMP that is being used.

• Community name—Defines an access environment for a group of NMSs. NMSs within the community are said to exist within the same administrative domain. Community names serve as a weak form of authentication because devices that do not know the proper community name are precluded from SNMP operations.

SNMPv2 Protocol Data Unit

SNMPv2 specifies two PDU formats, depending on the SNMP protocol operation. SNMPv2 PDU fields are variable in length, as prescribed by Abstract Syntax Notation One (ASN.1).

Figure 56-8 illustrates the fields of the SNMPv2 Get, GetNext, Inform, Response, Set, and Trap PDUs.

The following descriptions summarize the fields illustrated in Figure 56-8:

• PDU type—Identifies the type of PDU transmitted (Get, GetNext, Inform, Response, Set, or Trap).

• Request ID—Associates SNMP requests with responses.

• Error status—Indicates one of a number of errors and error types. Only the response operation sets this field. Other operations set this field to zero.

• Error index—Associates an error with a particular object instance. Only the response operation sets this field. Other operations set this field to zero.

• Variable bindings—Serves as the data field of the SNMPv2 PDU. Each variable binding associates a particular object instance with its current value (with the exception of Get and GetNext requests, for which the value is ignored).

Figure 56-8: SNMPv2 Get, GetNext, Inform, Response, Set, and Trap PDUs Contain the Same Fields

GetBulk PDU Format

Figure 56-9 illustrates the fields of the SNMPv2 GetBulk PDU.

Figure 56-9: The SNMPv2 GetBulk PDU Consists of Seven Fields

The following descriptions summarize the fields illustrated in Figure 56-9:

• PDU type—Identifies the PDU as a GetBulk operation.

• Request ID—Associates SNMP requests with responses.

• Non repeaters—Specifies the number of object instances in the variable bindings field that should be retrieved no more than once from the beginning of the request. This field is used when some of the instances are scalar objects with only one variable.

• Max repetitions—Defines the maximum number of times that other variables beyond those specified by the Non repeaters field should be retrieved.

• Variable bindings—Serves as the data field of the SNMPv2 PDU. Each variable binding associates a particular object instance with its current value (with the exception of Get and GetNext requests, for which the value is ignored).

Review Questions

Q—What are MIBs, and how are they accessed?

A—A Management Information Base (MIB) is a collection of information that is organized hierarchically. MIBs are accessed using a network-management protocol such as SNMP. They are comprised of managed objects and are identified by object identifiers.

Q—SNMP uses a series of _____ and ______to manage the network.

A—Gets and Puts. SNMP uses a Get object and a Put object to manage devices on a network such as get counters.

Q—Name three of the seven fields of the SNMP v2 GETBULK.

A—PDU Type, Request ID, Nonrepeaters, Max Repetitions, Variable Bindings (the variable bindings consists of variable object fields that make up the three remaining fields).

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