How to Save Your Control Network from the Gremlins of the Office Network

Overlapping VLANs, Rate Limiting and Port Security can Help Safeguard Your Control Network from Many Issues

Downers Grove, Illinois (June 15, 2006) - Three techniques - overlapped VLANs, rate limiting and port security―can help safeguard your control network from issues presented by the office network or office equipment such as Windows PCs.

All of these helpful features are available on Contemporary Controls CTRLinkÒ line of Ethernet managed switches (EICP, EISX, and EISB).

1) Overlapped VLANs - Bennet Levine, R&D Manager, says an overlapped VLAN is a special type of VLAN provided by managed switches that can allow one or multiple devices to have access to more than one VLAN. “That is, one or several devices can exist in multiple VLANs,” says Levine. “All other communications between the VLANs is blocked. For example, such an arrangement can allow a SCADA system to be shared between the office network VLAN and the control network VLAN, while blocking all other access between these VLANs. This enables SCADA communication with control network devices and allows office network devices to determine the control network status while protecting the rest of the control network from office network problems. Any office network issues will only affect the SCADA system. The rest of the control system will remain protected.”

[an error occurred while processing this directive] 2) Rate Limiting―Rate limiting is a managed-switch feature which can be employed to limit the bandwidth consumed by devices connected to specific ports of the switch. The user can specify a maximum bandwidth for each port of the switch. This can, for example, be used to limit the level of traffic being sent to a sensitive piece of equipment. Another use of this feature is to limit the traffic being sent to the control network from the office network. “In this way excessive traffic problems created by the office network, such as broadcast storms or directed message storms can be controlled by rate limiting,” explains Levine. “If you use Windows PCs in your control system, these devices can cause problems for the rest of the control system. It is recommended that rate limiting be used on all switch ports that connect to any Windows PCs. Rate limiting can also control the level of multicast messages.”

3) Port Security - This feature can be used to control which devices can communicate through specific ports of a managed switch. This can determine which office network devices can communicate with the control network, minimizing the problems presented by the office network. For example, port security can be used to only allow the plant manager and the engineering manager to have access to the control network from their computers on the office network. This would stop problems caused by any other office devices from reaching the control network.

Levine concludes by saying that these features protect your control network from problems which could cause production loss.

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