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The SMI defines the rules for describing management information by using ASN.1.
RFC 1902 (http://www.ietf.org/rfc/rfc1902) describes the SNMPv2 SMI and enhances the SNMPv1
SMI-specific data types by including:
Bit strings—Comprise zero or more named bits that specify a value.
Network addresses—Represent an address from a protocol family. SNMPv1 supports 32-bit IP
addresses, but SNMPv2 can support other types of addresses too.
Counters—Non-negative integers that increase until they reach a maximum value; then, the
integers return to zero. In SNMPv1, a 32-bit counter size is specified. In SNMPv2, 32-bit and 64-bit
counters are defined.
The SNMPv2 SMI specifies information modules, which include a group of related definitions. Three
types of SMI information modules exist:
MIB modules—Contain definitions of interrelated managed objects.
Compliance statements—Provide a systematic way to describe a group of managed objects that
must conform to a standard.
Capability statements—Used to indicate the precise level of support that an agent claims with
respect to a MIB group. An NMS can adjust its behavior towards agents according to the capability
statements associated with each agent.
The Get, GetNext, and Set operations used in SNMPv1 are exactly the same as those used in SNMPv2.
SNMPv2, however, adds and enhances protocol operations. The SNMPv2 trap operation, for example,
serves the same function as the one used in SNMPv1. However, a different message format is used.
SNMPv2 also defines two new protocol operations:
GetBulk—Used by the NMS to efficiently retrieve large blocks of data, such as multiple rows in a
table. GetBulk fills a response message with as much of the requested data as fits.
Inform—Allows one NMS to send trap information to another NMS and receive a response. If the
agent responding to GetBulk operations cannot provide values for all the variables in a list, the
agent provides partial results.
SNMP is a distributed-management protocol. A system can operate exclusively as an NMS or an agent,
or a system can perform the functions of both.
When a system operates as both an NMS and an agent, another NMS can require the system to:
Query managed devices and provide a summary of the information learned.
Report locally stored management information.
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