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NMS (Network Monitoring System)

An NMS system monitors your network. Most of these systems use SNMP. An NMS system takes the SNMP data and turns it into intelligible information you can use to pinpoint problems and to do troubleshooting. HP Open View, Nerve Center, What's Up Gold ($$$), Big Brother, Nagios (FREE) are all examples of SNMP-based NMS software packages.

Agent

An agent is a piece of software running on the local piece of equipment designed to monitor the local systems and notify an SNMP management server in the case of failures or when performance parameters are exceeded. Agents can be very advanced doing everhthing from killing and restarting Unix processes, rebooting the machine and sending e-mail. Agents are designed to send TRAPS to Master SNMP servers.

Master

An SNMP Master is a server that polls agents and is capable of receiving SNMP traps. SNMP Master servers use configuration files referred to generically as "MIBs".

Object

Objects are the smallest pieces of the SNMP system and are managed by an agent reading a MIB. Objects can be binary values, text, counters or other values. These are values either stored or set on the device.

Object Identifier (OID)

An object identifier (OID) is a number used to identify a specific value that can be SET or retrieved via a GET. OIDs are a string of numbers separated by periods (dots). The numbers specify which part of the Management Information Base is being referenced. In some cases, a human-readable alias can be used, which the agent or master will translate into the proper numerical OID.

Management Information Base (MIB)

The term MIB is used to describe a text file that is used to define a set of objects that can be retrived via an SNMP GET or that can be SET. The format of the entries in this file are governed by the Structure of Management Information standard. A MIB can be thought of as being organized into a tree-like structure. There is a standard "tree" structure of objects defined that all SNMP capable devices must support. The original set of standard MIB objects was eventually updated, so now you have MIB II (MIB two).

Structure of Management Information (SMI)

Structure of Management Information is a standard which defines how MIBs are defined and written. SMI defines how to construct an SNMP MIB.

Traps

One of several types of messages transmitted from an Agent to an SNMP Master. A trap is sent via UDP over IP using UDP port 162. Traps are sent to the Master when certain conditions configured in the agent are met, such as a temperature threshold being exceeded, voltage being too low, or CPU utilization exceeding a certain level. In SNMP v2/v2c a community string is used almost as a password for traps to authenticate the sender's information.

Polling

The SNMP master server should periodically send out queries to see if the remote equipment is still there. These queries can retrieve performance data and other values depending upon the capabilities of the remote agent. When querying equipment in SNMP v2/v2c a community string is used.

Community String

This is a text string used in SNMP versions 2 and 2c to uniquely identify one or more agents being monitored. This string functions both as a grouping identifier and as a security password. There is a community string used for reading (read-only community) from a device and a string used writing (read-write community) to the device. The default community strings for most equipment are set to "public" for the read string and "private" for the read-write string. On many HP printers, the default community "internal" is used (and can't be changed).

SNMP v1 Message Types