Simple Network Management Protocol (SNMP) …

SNMP Interoperability

As presently specified, SNMPv2 is incompatible with SNMPv1 in two key areas: message formats and protocol operations. SNMPv2 messages use different header and protocol data unit (PDU) formats than SNMPv1 messages. SNMPv2 also uses two protocol operations that are not specified in SNMPv1. Furthermore, RFC 1908 defines two possible SNMPv1/v2 coexistence strategies: proxy agents and bilingual network-management systems.

Proxy Agents

An SNMPv2 agent can act as a proxy agent on behalf of SNMPv1 managed devices, as follows:

• An SNMPv2 NMS issues a command intended for an SNMPv1 agent.

• The NMS sends the SNMP message to the SNMPv2 proxy agent.

• The proxy agent forwards Get, GetNext, and Set messages to the SNMPv1 agent unchanged.

• GetBulk messages are converted by the proxy agent to GetNext messages and then are forwarded to the SNMPv1 agent.

The proxy agent maps SNMPv1 trap messages to SNMPv2 trap messages and then forwards them to the NMS.

Bilingual Network-Management System

Bilingual SNMPv2 network-management systems support both SNMPv1 and SNMPv2. To support this dual-management environment, a management application in the bilingual NMS must contact an agent. The NMS then examines information stored in a local database to determine whether the agent supports SNMPv1 or SNMPv2. Based on the information in the database, the NMS communicates with the agent using the appropriate version of SNMP.

SNMP Reference: SNMPv1 Message Formats

SNMPv1 messages contain two parts: a message header and a protocol data unit (PDU). Figure 56-4 illustrates the basic format of an SNMPv1 message.

Figure 56-4: An SNVPv1 Message Consists of a Header and a PDU

SNMPv1 Message Header

SNMPv1 message headers contain two fields: Version Number and Community Name.
The following descriptions summarize these fields:

• Version number—Specifies the version of SNMP 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.

SNMPv1 Protocol Data Unit

SNMPv1 PDUs contain a specific command (Get, Set, and so on) and operands that indicate the object instances involved in the transaction. SNMPv1 PDU fields are variable in length, as prescribed by ASN.1. Figure 56-5 illustrates the fields of the SNMPv1 Get, GetNext, Response, and Set PDUs transactions.

Figure 56-5: SNMPv1 Get, GetNext, Response, and Set PDUs Contain the Same Fields

The following descriptions summarize the fields illustrated in Figure 56-5:

• PDU type—Specifies the type of PDU transmitted.

• 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 SNMPv1 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).

Trap PDU Format

Figure 56-6 illustrates the fields of the SNMPv1 Trap PDU.

Figure 56-6: The SNMPv1 Trap PDU Consists of Eight Fields

The following descriptions summarize the fields illustrated in Figure 56-6:

• Enterprise—Identifies the type of managed object generating the trap.

• Agent address—Provides the address of the managed object generating the trap.

• Generic trap type—Indicates one of a number of generic trap types.

• Specific trap code—Indicates one of a number of specific trap codes.

• Time stamp—Provides the amount of time that has elapsed between the last network reinitialization and generation of the trap.

• Variable bindings—The data field of the SNMPv1 Trap PDU. Each variable binding associates a particular object instance with its current value.

  标签：

  版权申明：本站文章部分自网络，如有侵权，请联系：west999com@outlook.com
  特别注意：本站所有转载文章言论不代表本站观点，本站所提供的摄影照片，插画，设计作品，如需使用，请与原作者联系，版权归原作者所有