There are several versions of SNMP, including:

All versions use SNMP managers, agents, and MIBs. Cisco IOS software supports the above three versions. Version 1 is a legacy solution and not often encountered in networks today; therefore, this course focuses on versions 2c and 3.

Both SNMPv1 and SNMPv2c use a community-based form of security. The community of managers able to access the agent's MIB is defined by an ACL and password.

Unlike SNMPv1, SNMPv2c includes a bulk retrieval mechanism and more detailed error message reporting to management stations. The bulk retrieval mechanism retrieves tables and large quantities of information, minimizing the number of round-trips required. The SNMPv2c improved error-handling includes expanded error codes that distinguish different kinds of error conditions. These conditions are reported through a single error code in SNMPv1. Error return codes in SNMPv2c include the error type.

Note: SNMPv1 and SNMPv2c offer minimal security features. Specifically, SNMPv1 and SNMPv2c can neither authenticate the source of a management message nor provide encryption. SNMPv3 is most currently described in RFCs 3410 to 3415. It adds methods to ensure the secure transmission of critical data between managed devices.

SNMPv3 provides for both security models and security levels. A security model is an authentication strategy set up for a user and the group within which the user resides. A security level is the permitted level of security within a security model. A combination of the security level and the security model determine which security mechanism is used when handling an SNMP packet. Available security models are SNMPv1, SNMPv2c, and SNMPv3.

The figure identifies the characteristics of the different combinations of security models and levels.

A network administrator must configure the SNMP agent to use the SNMP version supported by the management station. Because an agent can communicate with multiple SNMP managers, it is possible to configure the software to support communications using SNMPv1, SNMPv2c, or SNMPv3.