SETTING TRAPS

As mentioned before, a trap is an exception report similar to a change-of-state response from an I/O device. The manager establishes the trap in an agent. The agent monitors the situation and only reports to the manager if the trap is tripped. There are seven generic traps, but one is reserved for vendors for their specific application. The traps are as follows:

Generic Trap

Name, Number and Definition

coldStart (0)

Indicates that the agent has rebooted. All management variables will be reset; specifically, Counters and Gauges will be reset to zero (0). When a device is powered on, it sends this trap to its trap destination.

warmStart (1)

Indicates that the agent has reinitialized itself. None of the management variables will be reset.

linkDown (2)

Sent when an interface on a device goes down and identifies which interface.

linkUp (3)

Sent when an interface on a device comes back up and identifies which interface.

authenicationFailure(4)

Indicates that someone has tried to query the agent with an incorrect password.

egpNeighborLoss (5

Indicates that an Exterior Gateway Protocol (EGP) neighbor has gone down.

enterpriseSpecific (6)

Indicates that the trap is vendor specific.
As seen from this list, a much simpler approach can be taken to monitoring a device in the field besides polling. For example, a coldstart could indicate some unauthorized activity in the field that triggered the trap. The use of traps is no different from having the benefit of a remote annunciator in the field but without the added expense. By studying the vendor specific traps that are available from a particular product, more ingenious reporting is possible.

CONFIGURATION

Before commissioning a managed device in the field, its agent must be configured. This is not unlike the commissioning needed before installing a DeviceNet limit switch or photo-eye. With DeviceNet, you would use some tool or a program running on a laptop PC. Some devices will have a serial port that will support an ASCII terminal. If a terminal is unavailable, you could run a terminal emulation program on a PC. The advantage of this approach is that your network does not need to be up in order to commission the device. The second approach is to run a Telnet session over Ethernet. Of course, to do this the device must have its IP address already assigned. The screen on the PC will look the same but the network needs to be running. However, you could commission the device remotely from the control room with Telnet. In both of these cases, text screens are provided and the operator simply needs to fill in the blanks. The third approach is to use a web browser. This assumes that the managed device will serve up a web page for commissioning. With web technology, the screens are more colorful and data input is not restricted to simple command lines. Any of these approaches is possible but what data must be entered?
There are several parameters that must be set in the agent. The agent will consume an IP address for it to function as a management port. You might want to name the device, indicate its physical location and identify the person responsible for the device. You can even append a password to protect your settings. If traps are to be used, you need to identify the IP addresses of the managers that will receive the traps. There is usually space to list several IP addresses. What is significant here is that you need to know all this information before commissioning and to be careful not to reassign the master IP addresses, otherwise the traps will fail to find a manager. It would be a good idea to document all these parameters so a replacement device can be properly configured before putting the unit into service.

MANAGERS

Most of the discussion has been about agents and little about network management software. Command line programs can be used to poll agents and view responses, but the process is tedious since the operator needs to fully understand the structure of MIBs and each object’s syntax. There are several commercial software packages and some freeware packages that will poll agents, set traps and receive and display trap responses while providing a more convenient user interface. Since SNMP was developed before the Worldwide Web protocols were developed, much of the data that is displayed is text-based. Later versions of network management software take advantage of Windows functionality and provide more versatility such as trending. It will take an operator some time to learn the intricacies of the program but from one workstation, an operator can view all SNMP compatible devices.
Figure 4 — Typical Manager Screen

With increasing interest in making a web browser the default operator interface for a system, can SNMP data be displayed on a browser screen? Some newer managed devices have built-in web servers that can serve up SNMP data. The advantage of a built-in web server is that it provides a convenient method of configuring the device and, an opportunity to verify that the device is functioning by being able to access it from the web. The other advantage is that the managed device with its internal web server can operate stand-alone without the need for any network management software. The trick comes in when several managed devices are to be viewed from one browser. There is no consistency of data presentation from the various vendors of web-based managed devices. It is also inconvenient to remember all the various URLs that must be selected to view the individual managed devices.
For our industry, there is another approach. It is possible to have an OPC server running in the manager that understands the SNMP protocol and can query MIB data, but display the data in a format comfortable to the operator. If the operator is viewing a process automation screen to view instruments and controllers and alarms, the information from managed devices can be included within the same screen; thus, making for a neat uniform appearance. The operator does not need to run a totally different application program to monitor the health of the network. There are several vendors in our industry that provide such a product.

CONCLUSION

With more and more devices embracing Ethernet and Internet protocols, the addition of SNMP protocol support adds benefits to the device. Managed devices support the SNMP protocol and are called agents. Agents consist of a collection of managed objects that can be queried by a manager to determine the health of the network or the status of particular devices. By displaying this data in an easily understood format, operators and maintenance personnel, located at a central site, can monitor the performance of the entire network by observing selected devices and pinpointing potential problems before they occur. Although commercial and freeware network management software programs exist for this purpose, the trend is to use more web-based tools. SNMP is not restricted to just the management of switches and routers. Any industrial device can have SNMP support and could provide much aid in industrial applications.

References

Essential SNMP, Douglas R. Mauro & Kevin J. Schmidt, 2001, O'Reilly & Associates

DeviceNet Specifications, Open DeviceNet Vendors Association, Volume 1, Release 2.0, 1995

RFC 1157—A Simple Network Management Protocol (SNMP), Internet Engineering Task Force, 1990

RFC 1213—Management Information Base II, Internet Engineering Task Force, 1990