SNMPv1/SNMPv2c

SNMPv1/SNMPv2c Packet Format

As shown in Figure 1-3, an SNMPv1/SNMPv2c packet is composed of the version, community name, and SNMP Protocol Date Unit (PDU) fields.

Figure 1-3  SNMPv1/SNMPv2c packet format

The fields in an SNMPv1/SNMPv2c packet are as follows:

• Version: specifies the SNMP version. The value for SNMPv1 is 0 and for SNMPv2c is 1.

• Community name: used in the authentication of agents and NMSs. The community name is a character string and can be defined by users. There are two types of community names: read and write.

  • Read community names are used for the GetRequest and GetNextRequest operations.
  • Write community names are used for the Set operation.
• SNMPv1/SNMPv2c PDU: includes the PDU type, request ID, and binding variable list.

  • The SNMPv1 PDU includes GetRequest PDU, GetNextRequest PDU, SetRequest PDU, Response PDU, and Trap PDU.
  • The SNMPv2c PDU inherits the fields of SNMPv1 PDU and introduces the GetBulkRequest PDU.

  For simplification, the SNMP operations are described as the Get, GetNext, Set, Response, Trap, and GetBulk operations.

SNMPv1/SNMPv2c Operations

As shown in Table 1-1, SNMPv1/SNMPv2c defines six types of operations for exchanging information between the NMS and agents.

Working Mechanisms of SNMPv1/SNMPv2c

The working mechanisms of SNMPv1 and SNMPv2c are similar, as shown in Figure 1-4.

Figure 1-4  Basic operations

• Get

  In this example, the NMS intends to use the read community name public to obtain the value of the sysContact object on a managed device. The procedure is as follows:

  1. The NMS sends a GetRequest packet to the agent. The fields in the packet are as follows:

     • Version: SNMP version that the NMS is using
     • Community name: public
     • PDU type: Get
     • MIB object: sysContact

  2. The agent authenticates the SNMP version and community name in the packet. When authentication is successful, the agent encapsulates the sysContact value into the PDU of a response packet and sends the response packet to the NMS. If the agent fails to obtain the sysContact value, the agent returns an error message to the NMS.

• GetNext

  In this example, the NMS intends to use the community name public to obtain the value of the sysName object (next to sysContact) on a managed device. The procedure is as follows:

  1. The NMS sends a GetNextRequest packet to the agent. The fields in the packet are as follows:

     • Version: SNMP version that the NMS is using
     • Community name: public
     • PDU type: GetNext
     • MIB object: sysContact

  2. The agent authenticates the SNMP version and community name in the packet. When authentication is successful, the agent encapsulates the sysName value into the PDU of a response packet and sends the response packet to the NMS. If the agent fails to obtain the sysName value, the agent returns an error message to the NMS.

• Set

  In this example, the NMS intends to use the read community name private to set the sysName object on a managed device to HUAWEI. The procedure is as follows:

  1. The NMS sends a SetRequest packet to the agent. The fields in the packet are as follows:

     • Version: SNMP version that the NMS is using
     • Community name: private
     • PDU type: Set
     • MIB object: sysContact
     • Expected MIB object value: HUAWEI

  2. The agent authenticates the SNMP version and community name in the packet. When authentication is successful, the agent sets the sysContact object to the expected value and sends a response packet to the NMS. If the setting fails, the agent returns an error message to the NMS.

• Trap

  Trap is a spontaneous activity of a managed device. The Trap operation is not a basic operation that the NMS performs on the managed device. If a trap triggering condition is met, a managed device sends a trap to the NMS to notify the NMS of the exception. For example, when a managed device completes a hot start, the agent sends a warmStart trap to the NMS.

  The agent sends a trap to the NMS only when a module on the managed device meets the trap triggering condition. This mechanism reduces management information exchange between the NMS and managed devices.

Figure 1-5 shows the operations that are added in SNMPv2c.

Figure 1-5  Operations added in SNMPv2c

• GetBulk

  A GetBulk operation is equal to consecutive GetNext operations. You can set the number of GetNext operations to be included in one GetBulk operation.