SNMP Manager, Agent, Messages and Commands

what is SNMP
5 min

What is SNMP

In short,  SNMP (Simple Network Management Protocol) monitors and manages IP-connected devices.  It is no secret that more devices (or entities) are connecting to networks by the day which means the simple network management protocol is becoming increasingly important. 

The protocol is extremely important because it helps network administrators extract the information they need from devices for monitoring and managing them. 

In SNMP, there is a manager-agent relationship that runs the system. The manager is responsible for sending requests to the agents while the agents wait to get a request from the manager to tell them what to do and also replies to the manager with a response.

What is SNMP used for? 

In network monitoring and management, SNMP extracts variables and their respective variables about devices. Furthermore, it can offer more granular details about device attributes and behavior. For example; you can use SNMP to monitor variables about almost any device, for example; routers, printers, access points, hubs, NAS, UPS, and many others. 

Here’s a snapshot of some of the more granular information you can monitor on a device using simple network management protocol in comparison to standard network monitoring protocols. 

  • UPS devices: Alarms Present, Battery Status, Battery Voltage, Battery Current, Battery Temperature, Estimated Remaining Minutes, and Estimated Remaining Charge
  • Network Attached Storage (NAS) devices: 
    • Hard disk table: Hard Disks, Description, Status, Capacity, Info, Temperature, and Model: 
    • Volumes tables: Volumes, Description, Free Size, Total Size, Filesystem, and Status. 
    • Basic information: CPU Usage, CPU Temperature, Total Available Memory, Free Memory, Uptime, System Temperature, Number of Hard disks, Volumes, and Last Error Message.
  • Printers: Supply Units (Toner Cartridge, Drum Unit) and Supply Level:

Monitoring devices using the simple network monitoring protocol

Firstly, to start monitoring devices using SNMP, you need access to your device’s OIDs and MIBs. 

In today’s age, network monitoring software like Domotz contains pre-configured SNMP sensors. Furthermore, this software will automatically get the OIDs of your devices if they are supported. Moreover, this software also enables you to start monitoring your devices immediately, without requiring any configuration. 

Alternatively, if your network monitoring software, doesn’t already include pre-configured SNMP sensors or gather MIB information for you, you can use a MIB browser to find this information. 

SNMP Components 

These are the architecture components of the simple network monitoring protocol

  • The manager
  • The agents
  • A database of management Information.’
  • Managed objects
  • The SNMP itself 

What is the SNMP Manager?

The SNMP manager is the middleman between the human network administrator and the network system that is being managed. It ensures the SNMP agents check the value of certain defined variables in the MIB. The manager uses a method called Polling to facilitate this work, and all agents must respond to Polling. Users can initiate polling or it can be done automatically. Furthermore, in the case of Domotz network monitoring software, the manager is already embedded in Domotz.

What is the SNMP Agent?

The agent is in charge of the network communication. It runs on a managed device and is responsible for sending out traps, responding to queries, and answering requests from the SNMP manager. Once the agent receives a request, it acts upon it and then sends results to the manager.

Here is a pictorial representation of the architecture;


Simple network management protocol managers are responsible for managing their SNMP devices. Devices can include firewalls, switches, printers, routers, servers, CCTV cameras, load balancers and any other device which has SNMP capabilities in their specifications.


In short, commands act to simplify network management. Furthermore, these are the commands which can be used by and SNMP Manager to inquire an SNMP agent.

Here are some examples of basic commands;

GETGet values from the managed device.
GET NEXTGet the next OID Value.
GET BULKRecall bulk data.
TRAPThe trap command is sent by the agent to the manager.
RESPONSEThis is used to carry back the signal of actions directed by the manager.
What is SNMP and what are common commands


MIB is short for Management Information Base. This is a collection of information that is arranged in a hierarchy. All SNMP agents have a database where you can find information about all the devices it manages. MIBs are saved in a text file format. Furthermore, simulation tools, management tools, editors, and everyone who needs the information, can understand their formats.


OID is short for Object Identifiers. Their function is to identify objects that are in a MIB hierarchy. There are two types of managed objects;

●      Scalar objects define single objects

●      Tabular objects define multiple related objects.

OIDs are arranged in a hierarchy represented in the form of a tree. Each branch has a number and a name, and the path from the treetop to the point of interest forms the name of that point. 

Community Strings

Firstly, the “SNMP community string” is sort of like a user ID and password that allows access to an SNMP enabled device to inquire about its variables/properties. Furthermore, SNMP community strings are used only by devices which support the SNMPv1 and SNMPv2c protocol. Additionally, SNMPv3 uses username/password authentication, along with an encryption key.

Types of Community Strings

Read-only: With this string, you can only extract read-only information. The function does not let you modify the data, only read its values.

Read-write: With this string, you can carry out some other functions like reading the data value, modifying these values, and also reset them.

Trap community strings: This string receives SNMP traps from the device.

Above all, please note that many devices ship from the factory with a read-only community string set to “public” and read-write community string set to “private”. 

Traps and Trap Messages 

To start off, this section cover traps and messages. Firstly, SNMP traps send messages from an agent to a manager. Furthermore, they inform the manager, when they detect anomalies at the agent level. In addition, the trap message is different from other messages in the architecture. While other messages have to wait for the manager to send a status request before they can report a situation, the trap message can report itself without request by an instant trigger.

Here are some trap-type messages and their interpretations;

  • Cold Start: When an agent initializes its configuration table.
  • Warm Start: The message interprets an agent re-initializing its configuration table
  • Link Up: This message interprets a network adapter changing its state from down to up.
  • Link Down: Interprets a network adapter changing state from up to down.
  • Authentication Fail: The community name sent by the manager to the agent is invalid.
  • EGP Neighbor loss: The agent is unable to communicate with its Exterior Gateway Protocol (EGP).

What are the different SNMP Versions? 

SNMP has three versions;

  1. V1: Firstly, the first launch of SNMP v1 was in 1988. Moreover, it’s not in use much anymore which means it is closely becoming obsolete. Learn more about v1
  2. V2c: Firstly, SNMP v2 refers to the most commonly used of the three SNMP v2 versions, SNMP v2c. Moreover, it has an enhanced MIB structure element. Learn more about v2.
  3. V3: Security concerns drove the launch of SNMP v3. Furthermore, this version includes more effective features such as authentication and encryption. Ultimately, it has a greater focus on safety and security, to minimize eavesdropping and tampering. Learn more about v3.

Learn all about what is SNMP and how it works, SNMP v2 vs v3, how to find your SNMP OIDs, the differences in the OSI model vs TCP/IP model, and what is a network management tool.

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