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SNMP Tutorial Part 1: An Introduction to SNMP


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Since its creation in 1988 as a short-term solution to manage elements in the growing Internet and other attached networks, SNMP has achieved widespread acceptance. SNMP was derived from its predecessor SGMP (Simple Gateway Management Protocol) and was intended to be replaced by a solution based on the CMIS/CMIP (Common Management Information Service/Protocol) architecture. This long-term solution, however, never received the widespread acceptance of SNMP.

SNMP is based on the manager/agent model consisting of an SNMP manager, an SNMP agent, a database of management information, managed SNMP devices and the network protocol. The SNMP manager provides the interface between the human network manager and the management system. The SNMP agent provides the interface between the manager and the physical device(s) being managed (see the illustration above).

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SNMP is based on the manager/agent model
SNMP is based on the manager/agent model of a network management architecture.

The SNMP manager and agent use an SNMP Management Information Base (MIB) and a relatively small set of commands to exchange information. The SNMP MIB is organized in a tree structure with individual variables, such as point status or description, being represented as leaves on the branches. A long numeric tag or object identifier (OID) is used to distinguish each variable uniquely in the MIB and in SNMP messages.

SNMP uses five basic messages (GET, GET-NEXT, GET-RESPONSE, SET, and TRAP) to communicate between the SNMP manager and the SNMP agent. 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.

As you can see, most of the messages (GET, GET-NEXT, and SET) are only issued by the SNMP manager. Because 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.

The small number of commands used is only one of the reasons SNMP is "simple." The other simplifying factor is the SNMP protocol's reliance on an unsupervised or connectionless communication link. This simplicity has led directly to the widespread use of SNMP, specifically in the Internet Network Management Framework. Within this framework, it is considered "robust" because of the independence of the SNMP managers from the agents, e.g. if an SNMP agent fails, the SNMP manager will continue to function, or vice versa. The unsupervised communication link does however create some interesting issues for network alarm monitoring we will discuss more thoroughly in a later issue of our SNMP Tutorial.

Each SNMP element manages specific objects with each object having specific characteristics. Each object / characteristic has a unique object identifier (OID) consisting of numbers separated by decimal points (i.e., 1.3.6.1.4.1.2682.1). These object identifiers naturally form a tree as shown below. The MIB associates each OID with a readable label (i.e., dpsRTUAState) and various other parameters related to the object. The MIB then serves as a data dictionary or code book that is used to assemble and interpret SNMP messages.

MIB object identifier tree
The branch of the MIB object identifier tree.

When an SNMP manager wants to know the value of an object / characteristic, such as the state of an alarm point, the system name, or the element uptime, it will assemble a GET packet that includes the OID for each object / characteristic of interest. The element receives the request and looks up each OID in its code book (MIB). If the OID is found (the object is managed by the element), a response packet is assembled and sent with the current value of the object / characteristic included. If the OID is not found, a special error response is sent that identifies the unmanaged object.

When an element sends a TRAP packet, it can include OID and value information (bindings) to clarify the event. DPS remote units send a comprehensive set of bindings with each TRAP to maintain traditional telemetry event visibility. Well-designed SNMP managers can use the bindings to correlate and manage the events. SNMP managers will also generally display the readable labels to facilitate user understanding and decision-making.

This article in our series on the Simple Network Management Protocol (SNMP) examines the communication between managers and agents. Basic serial telemetry protocols, like TBOS, are byte oriented with a single byte exchanged to communicate. Expanded serial telemetry protocols, like TABS, are packet oriented with packets of bytes exchanged to communicate. The packets contain header, data and checksum bytes. SNMP is also packet oriented with the following SNMP v1 packets (Protocol Data Units or PDUs) used to communicate:

  1. Get
  2. GetNext
  3. Set
  4. Trap

The manager sends a Get or GetNext to read a variable or variables and the agent's response contains the requested information if managed. The manager sends a Set to change a variable or variables and the agent's response confirms the change if allowed. The agent sends a Trap when a specific event occurs.

SNMP Products:

snmp rtu
32-Point SNMP RTU
rtu power switching
8-port PDU (Power Strip) w/SNMP
security camera rtu
IP camera
w/SNMP
sfp fiber interface for SNMP
SNMP-over-Fiber
RTU
t1 interface for snmp reporting
SNMP-over-T1
RTU

The image below shows the packet formats. Each variable binding contains an identifier, a type and a value (if a Set or response). The agent checks each identifier against its MIB to determine whether the object is managed and changeable (if processing a Set). The manager uses its MIB to display the readable name of the variable and sometimes interpret its value.

SNMP Packet Formats
SNMP Packet Formats

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SNMP Tutorial by - July 21, 2010 - 20 pages
Publisher: DPS Telecom - Language: English - Free PDF Download
Average user rating: 3.9 out of 5 stars (17 reviewers)

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What Do You Want To Do With SNMP?

  1. Mediate SNMP and Other Protocols
  2. Display SNMP Alarm Data at Your NOC and in Your Web Browser
  3. Monitor Discrete, Analog, and Ping Alarms via SNMP
  4. Automatically Dispatch Repair Personnel
  5. Control Remote Equipment via SNMP
  6. Use Dial-Up Connections to Link SNMP Remotes to LAN
  7. Find the SNMP Alarm Monitoring Capacity That's Right for Your Size Site
  8. Use T1 Connection to Link SNMP Remotes to LAN

What Do You Want To Learn About SNMP?

  1. SNMP Tutorials - Learn More About SNMP
  2. 8 things you need to know before selecting an SNMP proxy agent to monitor your non-SNMP managed devices

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