An Introduction To MIB SNMP Concepts

SNMP MIB Concepts.

SNMP or Simple Network Management protocol, is an internet protocol defined by the Internet Engineering Task Force (IETF). SNMP has become one of the most commonly used protocols in monitoring applications. Its main function is to monitor network devices for alarms that require corrective action and/or human intervention.

In a typical SNMP network, there are several components that are linked together to give the company complete visibility of all their gear and sites.

SNMP Agent.

The agent is typically the device joined to the gear being monitored. For example, in a remote site you might be monitoring the temperature. If the temperature gets too high or too low, it could damage your gear. The temperature sensor is joined to an RTU, which acts as an agent in the system.

The agent takes readings from the sensor and relays the values to the manager. If the sensor or other gear is not SNMP compatible, the agent must mediate the protocol to SNMP for the manager.

SNMP Manager.

The job of the manager is to receive the values from the agent and display them in meaningful terms to the appropriate person for corrective action.

Types of agent-manager alarms.

There are several ways the agent and manager talk. TRAP messages are ones that are sent by the agent and signal a change-of-state (COS) alarm. If the temperature at a site rises above a predetermined level, the agent sends a TRAP message to the manager for human intervention.

GET alarms are sent by the manager, usually at regular, programmed intervals. They request status information from the agent. There are three types of GET alarms; GET, GET-NEXT, and GET-RESPONSE. Each message requires a slightly different response from the agent, but the basic function is the same.

Lastly, SET alarms are sent from the manager to the agent to initiate a change. For instance a SET alarm could tell the agent to open a door, set the temperature or other controlled inputs.

SNMP MIB.

The MIB, or Management Information Base, is an ASCII text file that describes SNMP network elements (any device in the network) as a list of data objects. It is essentially a dictionary of the SNMP language, where every object referred to in an SNMP message must be listed in the MIB.

The primary reason of the MIB is to translate numerical strings into readable text for humans. When an SNMP device sends a Trap or other message, it identifies each data object in the message with a number string called an object identifier. The MIB provides a text label called for each OID. Your SNMP manager uses the MIB as a codebook for translating the OID numbers into a human-readable display.

OID.

Object identifiers, or OIDs are used to describe SNMP network elements by using a number string. Each segment in the number string denotes a different level in the order, starting with one of the two companies that assign OIDs, all the way down to very single, company-unique, numbers. An OID may look like this:1.3.6.1.4.1.2682.1 as shown in the image below.

SNMP MIB tree
OIDs are formated using the MIB tree order, starting at the top and a period is used to separate the number identifier for each level.

How the MIB is structured.


The following keywords are used to define an OID in the MIB, also called a MIB object:

Syntax.

Defines the abstract data structure matching to the object type. It can take the form of binary numbers for discrete inputs (ON/OFF applications) or integers for analog inputs, which are more detailed than discretes.

Access

Defines whether the object value may only be retrieved but not modified (read-only) or whether it may also be modified (read-write). When a device notifies the RTU that there is an alarm, it is read only. Read-write describes values that also may be changed, such as a door is closed but the device can remotely open it.

Description

Contains a textual definition of the object type. The definition provides all semantic definitions needed for interpretation; it typically contains information of the sort that would be communicated in any commentary annotations associated with the object.