Solution Spotlight: Parallel Redundancy Protocol (PRP)

Network resiliency is becoming an important consideration for industrial automation applications.  As Ethernet networking continues to grow on the plant floor, uptime and reliability in the network is critical to maintaining operations.

Rockwell Automation has recently released a new ControlLogix Ethernet module, the 1756-EN2TP, which supports Parallel Redundancy Protocol, or PRP.

What is PRP?
A Parallel Redundancy Protocol (PRP) network offers a higher level of redundancy than DLR.  PRP is an international standard that is described in IEC 62439-3.  PRP
technology builds network redundancy into the end devices so that network infrastructure can be duplicated using standard components, such as managed and unmanaged switches.  Two local area networks (LANs) provide alternate paths for the traffic over independent LAN segments. However, the two LANs must share a similar topology.


PRP Terminology:

Example of a ControlLogix PRP network topology:

PRP provides a redundant network infrastructure. An important feature of PRP is that the two LANs do not meet, but connect at the PRP end nodes. Anend node that is connected to both LANs is called Doubly or Dually Attached Node or DAN. Both terms are fully accepted and used in the IEC standard.

Never connect LAN A and LAN B directly to each other. For example, never connect an infrastructure switch in LAN A to an infrastructure switch in LAN B.

What is a Redbox?
Next figure illustrates the addition of a Redundancy Box, or RedBox.  The RedBox allows non-PRP devices to be added to both LANs.  Devices that are connected to both LANs through a RedBox are called Virtual Doubly Attached Nodes, or VDANs. VDANs do not have media redundancy between the device and the RedBox, however, media redundancy exists on the LAN A/LAN B side of the RedBox. In this example, the Stratix 5400 switch is configured as a RedBox.

Note these Stratix 5400/5410 switch firmware sets are required for Redbox support:

  • Stratix 5400 Firmware 15.2(2)EA1 to 15.2(5)EA.fc4
  • Stratix 5410 Firmware 15.2(2)EA1 to 15.2(5)EA.fc4

Also, this Rockwell Automation Knowledge Base article discusses how to configure a Stratix 5400/5410 as a Redbox.

 Parallel Redundancy Protocol (PRP) Topology, with a Stratix 5400 as a RedBox

Another important concept for products with PRP technology is that each have two ports. Each port connects to a separate PRP LAN that then connects to two independent sets of network infrastructures.  You cannot use these ports as two Network Interface Cards (NICs) connected to two different subnets.  Each 1756-EN2TP module has only 1 unique MAC and has only one IP address.  The 1756-EN2TP Ethernet Communication module cannot be used as part of a DLR ring, because the 1756-EN2TP supports PRP but not the DLR protocol.

Design Implementation Guide:

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