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How Does SNMP Work And How to Configure It?

October 4, 2019

This article will guide you through what is SNMP, how SNMP works, how to configure it, and frequently asked questions about SNMP.

What Is SNMP?

SNMP (Simple Network Management Protocol) is an application-layer protocol that contains three versions, including SNMPv1, SNMPv2c and SNMPv3. All the versions provide a standardized framework and a common language for monitoring and managing devices in a network.

Devices that support SNMP include network switches, routers, printers and so on. Those devices are produced by different vendors and their management interfaces (such as CLI) vary greatly, which makes network management more complicated. SNMP is designed to solve the problem. It provides a unified interface to implement unified management on different devices from different vendors, which greatly simplifies network management.

SNMP Components

There are four main SNMP components of an SNMP management model, including NMS (network management system), SNMP agent, MIB (management information base) and managed object. Each managed device includes agent access, MIB, and several management objects.

NMS acts as a manager on a network, and it may be a computer or server that runs some kind of SNMP software to monitor and manage network devices.

Agent is a process that runs on a managed device to receive the request from NMS and then return the answers to the NMS.

MIB refers to a data base that contains the variables that the managed device maintains (information could be query and set by agent).

Management object is an object that will be managed. It could be a hardware component (like an interface board) or parameters configured for the hardware or software (like a route selection protocol).

The NMS interacts with the agent on a managed device. After the NMS sends a command to the agent, the agent will perform operations on the MIB in the managed device.

How SNMP Works?

SNMP Working Principles

Here uses SNMPv2c to explain SNMP working principles. It performs the following operations to retrieve data, modify SNMP object variables and send notifications.

Note that SNMPv1 does not support GetBulk and Inform operations.

Get GetNext GetBulk Set Response Trap Inform
It’s a request sent by the NMS to the managed device. And it’s performed to retrieve one or more values from the MIB. It’s similar to the GET. But it usually retrieves the value of the next OID (Object Identifier) in the MIB tree. It’s used to retrieve a mass of data from large MIB table. It’s performed by the NMS to modify the value of the managed device. It’s performed by the agent in response to the GetRequest, GetNextRequest, GetBulkRequest and SetRequest operations. This operation is initiated by the agent. It’s used to notify the NMS of a fault or event occurring on a managed device. This operation is initiated by the agent. It’s similar to the TRAP, but after the agent sends an inform request, the NMS must send an InformResponse packet as a response to the agent.

Figure 2 SNMPv2c Working Principles.

When NMS sends a Get/GetNext/GetBulk/Set request packet to the agent, the agent authenticates the SNMP version and community name first. Then the agent sends the corresponding value as a response packet to the NMS when the authentication is successful. If the agent fails to obtain the corresponding value, it returns an error message to the NMS. Note that, the GetBulk operation is equal to consecutive GetNext operations. Users can set the numbers of GetNext operations that are included in one GetBulk operation, without the need for repeated GetNext operations.

Figure 3 Trap/Inform process of SNMPv2c application.

Trap and Inform are spontaneous activities of the managed device. When a trap triggering condition occurs, the managed device sends a trap to the NMS for notification. Inform is similar to the Trap, but the Inform requires a confirmation from the NMS while the Trap does not. If the managed device doesn’t receive the confirmation, it saves the Inform in the buffer or repeats the Inform operation until the NMS returns a confirmation. Once the number of repeated sending reaches the maximum limit, the managed device will record a log for the Inform request. FS offers 1G/10G switches, 40G switches that supports SNMP. Please visit FS.COM for more information.

How to Configure SNMP?

SNMP uses a central computer with SNMP software installed to manage network switches. SNMP configuration is not that complicated as most network switches on the market nowadays, be they Gigabit switch or 40G switch, all support SNMP that provides a unified and easy way to manage those switches. Take SNMPv2c configuration as an example, the process includes:

A) Configure IP address on the computer and the managed switches.

B) Enable SNMP.

C) Configure the access rights to enable the computer to manage the specified switches.

D) Verify the configuration result.

More details about SNMP configuration, please visit SNMP Configuration on FS 3900 Series Switches.

FAQs about SNMP

Q: What should I do if NMS failed to receive the trap?

A: In the default configuration, not all Trap are enabled. In system view, users can:

  • View the feature list that has enabled Trap by showing snmp-agent trap

  • Open the corresponding Trap by snmp-agent trap enable feature-name

  • Open all the Trap by snmp-agent trap enable

Q: SNMPv1 vs SNMPv2c vs SNMPv3, what are the differences?

A: The following table shows a clear picture of the comparison among these three types of SNMP:

SNMPv1 SNMPv2c SNMPv3
Access control Based on the community names and MIB view Based on the community names and MIB view Access control based on the user, user group, and MIB view
Authentication and privacy Based on the community name Based on the community name Authentication mode: MD5/SHA Encryption mode: DES56/AES128/AES192/AES256/3DES
Trap Support Support Support
Inform Not Support Support Support
GetBulk Not Support Support Support
Application Suitable for small networks with low security requirements Suitable for SMB and larger networks, with low security requirements Suitable for large and super-giant enterprises with strict security requirement

Summary

SNMP is one of the widely accepted network protocols to manage and monitor network elements. For SNMP to work, one has to take into consideration SNMP components and SNMP working principles. Most of the professional–grade network elements come with bundled SNMP agent.

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