An Introduction To SNMP MIB Files

In this article, you'll learn what MIBs are and you'll have a quick SNMP guide.

Management Information Base, or MIB, is a formatted text file that lists the data objects used by a particular piece of SNMP equipment.

How Does MIB Work?

1. The manufacturer of your device will supply you with a MIB file.
2. You'll load that file into your SNMP manager through the interface it provides.
3. Your SNMP manager will use the provided MIB data to interpret the incoming messages from your new device.

When you buy a device that uses SNMP (for example, a managed switch), you'll tell it to send messages to your central SNMP manager. But there are tens of thousands of different SNMP devices and your manager doesn't natively understand each one.

The manufacturer of your managed device will supply you with a file (usually a download from their website) that you'll load ("compile") into your SNMP manager (If you've ever installed a device driver on a PC, you understand this concept). Without the management information base for message translation, communication simply won't happen.

The manager uses MIB files from each agent to decode the OIDs ("Object Identifiers", which are strings of numbers) and translate them into meaningful information.

What is an SNMP Compliant MIB?

A MIB contains definitions and info about the properties of managed resources and the services that the agents (devices) support. The features of resources, as defined in a Management Information Base, are called "managed objects" or "management variables".

A management station gets and sets objects in the Management Information Base, and an agent notifies the management station of events using messages called (somewhat oddly) "traps". All message exchanges between the management station and its agents take place using SNMP. The Management Information Base at the management station contains network management info. This info is taken from the MIBs of all the managed parts in the network.

What are MIB files used for?

Your SNMP manager needs the MIB in order to process messages from your devices. This is a critical step in network monitoring. The Management Information Base is also your best guide to the real capabilities of an SNMP device. You need to be able to read the MIB so that you can have a good idea of what assets you do have.

How Can MIB Objects Be Updated?

Updating MIB objects is essential for maintaining and enhancing the functionality of network monitoring systems, especially when using devices like SNMP-based RTUs. To update MIB objects:

1. Download the New MIB File: If you're adding new devices or need updated MIB objects for existing ones, first obtain the updated MIB file. Manufacturers often provide MIB files for their devices. For DPS Telecom devices, MIBs can be found through their support or product pages.
2. Install the MIB on Your SNMP Manager: Once you have the updated MIB file, it must be loaded into your SNMP manager software. Follow the specific installation process of your SNMP manager.
3. Modify the MIB as Necessary: In some cases, MIB objects might need customization to align with your specific network monitoring needs. You can edit MIB objects to reflect custom traps or specific values for better management of your network elements. This is done by editing the MIB using text editors or MIB management tools, making sure that the object identifiers (OIDs) match your device's requirements.
4. Upload or Apply the Updated MIB: After installing or updating the MIB file on your SNMP manager, it's essential to restart or refresh the manager's SNMP service. This verifies that the new MIB objects are active and that the manager can query and understand the updated object identifiers.
5. Test and Validate: Once the MIB has been updated, it's important to verify that the SNMP manager can successfully communicate with the device using the new MIB objects. You can trigger SNMP queries or traps and confirm that the SNMP manager is interpreting them correctly based on the updated MIB.

By updating your MIB objects, you can uphold your network's ability to monitor infrastructure, whether that's handling new devices, supporting protocols, or just basic efficient functioning.

What are some examples of objects monitored by MIB?

The MIB monitors various objects that reflect the operational status and performance of network devices. Examples include interface status (ifOperStatus), which indicates whether a network interface is up or down. Another example is CPU utilization (hrProcessorLoad), which tracks the percentage of CPU usage on a device to help identify potential performance issues. These monitored objects are important for maintaining the health and efficiency of network operations.

What are the differences between scalar and tabular MIBs?

Scalar and tabular MIBs differ primarily in how they represent data within the MIB structure. Scalar MIBs define single-instance objects. This means each object has only one instance and thus represents a single piece of information. For example, an object that holds the system uptime (sysUpTime) is scalar because there is only one uptime value for the entire system.

Tabular MIBs define multiple-instance objects, which are organized in tables. Each entry in the table corresponds to a row that can have multiple instances of data, such as a list of network interfaces where each interface has attributes like status, speed, and IP address. These tabular MIBs are used for representing repetitive information in a structured way. This allows multiple related data points to be accessed through indexed rows.

How do I look at a MIB?

One of the best tactics for addressing MIB problems is to simply read through the file. As the file is just ASCII text, you can view it in any word processor or text editor (even Notepad). Some manufacturers provide grouped files in binary format, but those aren't readable. You want the raw ASCII version of the Management Information Base file.

An easier way to get started with Management Information Bases can be to use MIB browsers. These are software programs that allow you to view it in a more friendly format, commonly a tree containing each MIB object.

Do you need to download DPS' MIBs, Cisco MIBs, or even Chronos MIBs? We have an extensive library with a list of MIBs supported by different vendors. There, you can download these files for free.

What is SNMP?

The Simple Network Management Protocol, or SNMP, is a standard protocol, developed by The Internet Engineering Task Force (IETF).

SNMP is used for monitoring and management of network devices that span firewalls or embedded devices (known as SNMP agents).

SNMP exposes management data in the form of variables on the managed systems. These describe the system configuration. These variables can then be sorted (and sometimes set) by managing applications.

What operations are used by SNMP?

SNMP uses several key operations to manage and monitor network devices:

• GET: Retrieves the value of a specific object identified by its object identifier (OID) from a network device.
• GETNEXT: Retrieves the value of the next object in the MIB tree, allowing sequential retrieval of values.
• GETBULK: Efficiently retrieves large blocks of data, commonly used in SNMPv2c and SNMPv3 to reduce the number of GETNEXT requests.
• SET: Modifies or sets the value of a specific object identified by its OID on a network device.
• TRAP: Asynchronously notifies the network management system of significant events or changes in a network device.
• INFORM: Similar to TRAP, but includes an acknowledgment from the receiver to make sure that the notification was received.
• RESPONSE: Sent by an SNMP agent in reply to a GET, SET, GETNEXT, GETBULK, or INFORM request, containing the requested data or status of the operation.

These operations enable SNMP to effectively monitor and manage network devices by retrieving, setting, and reporting data.

What is the structure of SNMP messages?

SNMP messages are structured with key components that aid communication between network management systems and devices. Each message includes:

• A version identifier.
• A community string or security parameters (depending on the SNMP version).
• A Protocol Data Unit (PDU).

The PDU contains the operation to be performed (such as GET or SET), along with fields like request ID, error status, and error index. It also includes variable bindings (VarBinds), which are pairs of Object Identifiers (OIDs) and their corresponding values, specifying the data to be accessed or modified. This structured format ensures precise and efficient network management.

What versions of SNMP are available?

There are several SNMP versions, but the most common by far are SNMP v1, v2c, and v3.

The SNMP v1 network management architecture contains:

• Network Management Station (NMS) - Workstation that hosts the network management app.
• Network management application - Polls management agents for information and provides control info to agents.
• Management agent(s) - Provides info contained in the Management Information Base to management applications and may accept control information.
• Community strings as the main security feature.

SNMP v2c specs include the following new capabilities:

• Manager to manager communication to support multiple / distributed managers and mid-level managers.
• Better security (known as "Secure SNMP") by specifying three layers of security.
• Improved efficiency and performance through the addition of bulk transfers of data.
• SNMP Inform message type includes delivery confirmation, an improvement over the simple SNMP trap message.

SNMP v3 specs include the encryption required by security-conscious organizations. Most networks have large amounts of SNMP gear already, though, and must find another way to deploy v3 for security?

With an SNMPv3 mediator, it's possible to upgrade older SNMP-enabled devices to SNMPv3. It's pretty simple equipment in concept, but only offered by a few manufacturers.

Download White Paper: "How to Read and Understand the SNMP MIB"

Part 2: Tactics for resolving common MIB problems

Part 3: Why is the MIB important?