Difference between revisions of "Simple Network Management Protocol"

Revision as of 11:50, 14 September 2007

Used for monitoring network devices and encompassing a set of standard protocols. SNMP data contains information describing the network device configuration, and variables describing the state of the device. SNMP can be used to both read and write to the network device, such as reading the configuration and then modifying the configuration. SNMP is most commonly used for monitoring.

SNMP works by sending messages, called protocol data units (PDUs), to different parts of a network. SNMP-compliant devices, called agents, store data about themselves in Management Information Bases (MIBs) and return this data to the SNMP requesters. SNMP uses five basic messages (GET, GET-NEXT, GET-RESPONSE, SET, and TRAP) to communicate between the SNMP manager and the SNMP agent. There are variations to account for additional message types (see the six listed below) based on the version of SNMP being used.

Information from network devices may be retrieved via

• GET
• GETNEXT
• GETBULK

Data may be streamed using

• TRAP
• INFORM

Data may be written using

• SET

description

• The read command is used by an NMS to monitor managed devices. The NMS examines different variables that are maintained by managed devices.
• The write command is used by an NMS to control managed devices. The NMS changes the values of variables stored within managed devices.
• The trap command is used by managed devices to asynchronously report events to the NMS. When certain types of events occur, a managed device sends a trap to the NMS.

SNMP uses the default UDP port 161 for general SNMP messages and UDP port 162 for SNMP trap messages. SNMP uses an unsupervised or connectionless communication link.

The GET and GET-NEXT messages allow the manager to request information for a specific variable. The agent, upon receiving a GET or GET-NEXT message, will issue a GET-RESPONSE message to the SNMP manager with either the information requested or an error indication as to why the request cannot be processed. A SET message allows the SNMP manager to request a change be made to the value of a specific variable in the case of an alarm remote that will operate a relay. The SNMP agent will then respond with a GET-RESPONSE message indicating the change has been made or an error indication as to why the change cannot be made. The SNMP TRAP message allows the agent to spontaneously inform the SNMP manager of an "important" event.

(GET, GET-NEXT, and SET) are only issued by the SNMP manager. The TRAP message is the only message capable of being initiated by an SNMP agent, it is the message used by DPS Remote Telemetry Units (RTUs) to report alarms. This notifies the SNMP manager as soon as an alarm condition occurs, instead of waiting for the SNMP manager to ask.

Simple Network Management Protocol (SNMP) is a widely used protocol for monitoring the health and welfare of network equipment (eg. routers), computer equipment and even devices like UPSs. Two versions of SNMP exist: SNMP version 1 (SNMPv1) and SNMP version 2 (SNMPv2). Both versions have a number of features in common, but SNMPv2 offers enhancements, such as additional protocol operations. Standardization of yet another version of SNMP—SNMP Version 3 (SNMPv3)—is pending.

Variables accessed by SNMP are organized in hierarchies. These hierarchies, and other metadata, are described by Management Information Bases (MIBs). An SNMP-managed network consists of three key components: managed devices, agents, and network-management systems (NMSs).

SNMP must account for and adjust to incompatibilities between managed devices. Different computers use different data representation techniques, which can compromise the capability of SNMP to exchange information between managed devices. SNMP uses a subset of Abstract Syntax Notation One to accommodate communication between diverse systems.

SNMP Messages

SNMP messages may be initiated by either the network management system (NMS) or by the network element.

An SNMP TRAP is a message which is initiated by a network element and sent to the network management system. For example, a router could send a message if one of it's redundant power supplies fails or a printer could send an SNMP trap when it is out of paper.

An SNMP GET is a message which is initiated by the network management system when it wants to retrieve some data from a network element. For example, the network management system might query a router for the utilization on a WAN link every 5 minutes. It could then create charts and graphs from that data, or it could warn the operator when the link was overutilized.

An SNMP SET is a message which is initiated by the NMS when it wants to change data on a network element. For example, the NMS may wish to alter a static route on a router.