What Is SNMP?

SNMP, the Simple Network Management Protocol, is the IETF standard for reading status from network devices and, where allowed, changing their settings. Almost every managed switch, router, firewall, UPS and printer supports it, and so do Windows and Linux computers through an agent service.

There are three parts. The agent is software on the managed device. The manager, often called a network management system or NMS, is the monitoring software that collects the data. The MIB, or management information base, is the catalog of values the agent can report, each named by a numeric object identifier (OID) such as 1.3.6.1.2.1.1.3.0 for the time since the device last restarted.

The manager polls: it sends Get, GetNext or GetBulk requests to UDP port 161 on the agent and receives the values back, typically every one to five minutes. When something important happens between polls, such as a port going down, the agent sends an unsolicited notification called a trap to the manager on UDP port 162. A Set request writes a value, for example to disable a port, if the agent allows write access.

Three versions are in use, and the difference that matters most is security.

A community string works like a shared password, and the factory defaults “public” for read and “private” for write are widely known. Because v1 and v2c send it in clear text, anyone who can capture traffic can read it. SNMPv3 adds named users with three security levels: noAuthNoPriv, authNoPriv and authPriv. Use authPriv, which both authenticates and encrypts, wherever the devices support it.

Standard MIBs give every vendor’s equipment a common baseline, and private MIBs from each manufacturer add device-specific values. Typical data from a managed switch includes:

In a plant, SNMP is how the network reports on itself. A redundant ring or backup link keeps production running when a cable is cut, which also means nobody notices the fault unless a trap reaches someone. Pointing every managed switch at a network monitoring server, or at a SCADA package with an SNMP driver, turns a silent failure into an alarm. Our managed industrial switches support SNMP, with SNMPv3 on most models, and many also report status over Modbus TCP so a PLC or SCADA system can read it directly.

Keep SNMP traffic on a management VLAN, allow only the monitoring server to query the agents, turn write access off unless you need it, and change or remove the default community strings. Our guide to choosing between managed and unmanaged industrial switches covers when that monitoring is worth having.

A small amount of planning at commissioning saves a lot of guesswork later. The steps below suit most plant networks, whichever monitoring software you use.

SNMP is rarely the only source of data. Syslog, usually on UDP 514, carries text log messages that explain why something happened, while SNMP reports the counters and states that show what happened. Larger IT networks are adding NETCONF and streaming telemetry for configuration and high-rate data, but on industrial equipment SNMP remains the protocol most devices support, which is why it is still the common baseline across switches, UPSs and computers from different manufacturers.

We supply managed industrial switches and edge computers with SNMPv3 users, trap destinations and management VLANs set to your monitoring plan as standard, so faults reach your team from the first day of operation.