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The Basics of Using SNMP Devices

An SNMP device is, quite simply, a device that is managed using the Simple Network Management Protocol (SNMP). SNMP is an open-source protocol, meaning that any manufacturer can utilize it.

While there are many protocols that devices can use, SNMP is one of the most common protocols used, because of its ease of use and access. This allows for managed devices to talk across your network, even if different manufacturers produced those SNMP devices.

Let's take a look at the basics of SNMP devices and why being able to handle them is so important.

SNMP Enabled Device Types

Types of SNMP devices include switches, bridges, routers, access servers, computer hosts, hubs, and printers. These are devices that are part of your SNMP network and relay important information via SNMP about the status of your remote sites.

Perhaps the most important device for remote alarm monitoring and control is the general-purpose remote telemetry unit (RTU). An RTU can monitor network performance, manage multiple devices, and send important status information to a master or person responsible to fix any issue. It can be used alone to monitor a single site or joined to a manager or master station if your network has multiple sites with multiple RTUs.

It is important to note that not all RTUs have SNMP compatibility. If your network is SNMP-based, it is important to find an SNMP-compatible RTU.

MIB flow chart
A basic manager agent, and generator relationship. The generator, or other gear, sends a COS alarm to the RTU. The RTU then translates the alarm into SNMP and relays it to the manager to start corrective actions.

A manager, or master, is another very important SNMP device, but it is not always needed. When a company needs to monitor a lot of remote sites, they may use a manager. It joins all the RTUs and equipment they monitor into one, easy to monitor, streamlined platform. This allows the company to monitor a large amount of gear remotely from one central office.

A Basic SNMP Manager, or Master, Can be Limited

A major drawback when you use SNMP managers is that they can be limited to SNMP devices only. For a network of devices that use other protocols (such as TL1, TBOS, TABS, or ASCII text messages), other alarm masters will be needed to manage those devices. This may add extra screens to your network monitoring system. It may also require more staff and training to monitor.

A good network management station will be able to handle multiple varieties of protocols so that all information about your sites can be accessed from a central information hub.

Multi-Protocol Masters can Integrate SNMP Devices

A multi-protocol master alarm system can join SNMP devices with other devices to present a common interface. All the events appear on a single management and control interface. This gets rid of the complexity of a lot of screens, additional operators, and training. This can have significant cost advantages for your company.

SNMP Protocol Flexibility

The SNMP protocol is quite simple in its design and gives manufacturers a lot of latitude in adding SNMP on their units. This can be challenging for integration because each SNMP device can have large differences from other manufacturers devices with respect to its SNMP profile.

Most manufacturers will have a management information base (mib) that contains the object identifiers needed to access their devices. Something to watch out for is whether a device manufacturer provides their mibs or sells them separately. It could save you from setting up an SNMP network, only to realize that the manufacturer of the device you are polling sells their mibs at a very high price. Without device mibs, it is nearly impossible to know how to successfully perform SNMP operations on a device.

Versions of SNMP

For different network management systems there are different versions of SNMP. Most devices support versions 1 and 2c, but it is important to check what version of SNMP your devices can support. The main versions of SNMP are SNMPv1, SNMPv2c, and SNMPv3.

Version 1 is the oldest, and easiest to setup. It only requires that requests and traps have a community attached, which acts in a similar way to a password, however it is in plain-text. The biggest downsides to it are that it has little security, and only supports 32-bit counters. Generally though, if your devices only need to be read, and have no set privileges available, this isn't a major concern.

Version 2c is virtually identical to version 1, except it adds support for 64-bit counters.

Version 3 adds extra security features in the form of encryption for the packets sent and authentication. This version is more difficult to set up, but if security is a concern this is what this version was meant to address.

Click here for further reading on different SNMP versions.

Now that you have a better understanding of the basic differences between SNMP versions, you can verify which version you should look into supporting across your network.

An SNMP Device Reports Events

There are two main types of messages that an SNMP device can report. A manager can issue a "GET" request to an RTU (also sometimes called an SNMP agent). A "GET" message asks the RTU to report a value back to the manager so that it can access the status of a site or piece of gear. There are several types of these messages; "GET", "GET-NEXT" and "GET-RESPONSE".

Conversely, an SNMP device, usually an RTU, can send a message called a TRAP whenever a change-of-state (COS) event occurs. These TRAP messages are sent to a manager, which converts the TRAP into a remote alarm for management and control. Managers typically require object identifiers in order to perform requests.

The third type of message exists, called a "SET" message. The "SET" request is only issued from a manager to an RTU and is intended to change a discrete or analog input. For instance, a "SET" request may require the RTU to turn on a generator or set a thermostat to a specific temperature.