PRP – PARALLEL REDUNDANCY PROTOCOL

What is PRP?
PRP — Parallel Redundancy Protocol
PRP (Parallel Redundancy Protocol) is a redundancy protocol for Ethernet based networks requiring high availability and a short switchover time, as for example protection systems at electrical substations.
Unlike common redundancy protocols like RSTP, PRP reacts to any network component failures seamlessly (without recovery time) and is invisible to the application.
- PRP is a network redundancy protocol for Ethernet networks standardized by IEC 62439–3:2016
- Together with HSR, it has been adopted in the IEC 61850 standard for substation automation
- It is used in critical communication networks that request high availability and short switchover times
- It it is particularly fast, making it suitable for applications where the recovery time of commonly used protocols such as the Rapid Spanning Tree Protocol (RSTP) is too long
- It is independent of the application-protocol and can be used by most Industrial Ethernet protocols in the IEC 61784 suite
jump to Schema | jump to How it works | jump to PRP vs HSR |
Advantages and Disadvantages of PRP
- It provides seamless failover (zero time recovery) against failure of any network component
- It is independent of & invisible to the application-protocol and can be used by most Industrial Ethernet protocols
- It can be connected without single point of failure
- The same nodes can be built to be used in both PRP and HSR networks, since both rely on the same duplicate identification mechanism
- Flexible network architecture
The main disadvantage is that it doubles the cost of installed infrastructure (switches and other components) in comparison to single, non-redundant networks.
When to Use PRP
PRP is specially suited for applications requiring highly flexible network structures with high-speed switchover times close to zero.
How it Works
The basic approach of PRP network redundancy is to have two independent active paths between two devices. In practical terms, this means that each node is connected to two separated, parallel Local Area Networks (LANs). This allows the source nodes (senders) to transmit two copies of each packet, one over each network. When a destination node receives a package, the protocol makes sure that it will accept the first copy of the data packet and discard the second copy. In this way the destination node will always receive at least one packet as long as one of the two networks is operational and no frames are lost.
The two independent parallel networks can be of arbitrary topology as long as they are similar. Furthermore they should have no links between them and be fail-independent to avoid common mode failures. The networks can be based on redundant schemes (MRP, RSTP etc.) or have no redundancy at all.
PRP Redundancy Networks Explained
The PRP devices are named DANP (doubly attached node using PRP), while standard devices are named SAN (singly attached node).
One DANP has two Ethernet ports attached to two separate local area networks with DANP and SAN messages living together in one LAN. As LAN 1 and LAN 2 are not connected, two Ethernet interfaces of a node use the same MAC address. In this way, PRP is a layer 2 redundancy, allowing higher layer network protocols to operate without requiring modification.

PRP vs HSR — Which one is better?
All in all, the choice between HSR and PRP should depend on the specific project, as the project will determine which drawbacks and advantages weigh more or less. As a rule, PRP network redundancy is more expensive, but also more flexible than HSR.
PRP redundancy usually doubles the cost of a single non-redundant network, which makes the implementation more expensive than many other redundancy protocols. A High-availability Seamless Redundancy (HSR) network can also provide the same level of redundancy and will cost less unless you’re planning on keeping replacement infrastructure anways.
However, PRP networks provide greater flexibility in terms of network topology and allow to connect (PRP) “unaware” nodes without a RedBox (Redundancy Box), which would be required by HSR. Yet, these singly attached nodes can only communicate with devices of the same LAN, not taking full advantage from the redundancy.
Other minor disadvantages of HSR are potential technical limitations in relation to traffic, latency and bandwidth. Still, these problems are not very frequent and can be counteracted with different methods.
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Network Communication
- HSR — High-availability Seamless Redundancy
- PRP — Parallel Redundancy Protocol
- Ethernet
- MAC — Medium Access Control
- VLAN — Virtual Local Area Network
- IEC 62439–3
- Network Redundancy
HSR (High-availability Seamless Redundancy) is a redundancy protocol for Ethernet networks requiring short reaction times and high availability, as for example protection systems at electrical substations.
Unlike common redundancy protocols like RSTP, HSR reacts to any network component failures seamlessly (without recovery time) and is invisible to the application.
PRP (Parallel Redundancy Protocol) is a redundancy protocol for Ethernet based networks requiring high availability and a short switchover time, as for example protection systems at electrical substations.
Unlike common redundancy protocols like RSTP, PRP reacts to any network component failures seamlessly (without recovery time) and is invisible to the application.
Ethernet is a family of wired computer networking technologies commonly used in local area networks (LAN) and also wide area networks (WAN).
Over time, Ethernet has largely replaced competing wired LAN technologies by providing higher bit rates, a greater number of nodes, and longer link distances and decent backward compatibility.
The Medium Access Control (MAC) sublayer provides flow control and multiplexing for the transmission medium to control the hardware that interacts with the wired, optic and also wireless transmission media in the IEEE 802 LAN/MAN data link layer.
The MAC is accompanied by the LLC sublayer, which provides flow control and multiplexing for the logical link (i.e. EtherType, 802.1Q VLAN tag etc.)
A Virtual Local Area Network (VLAN) is a is a subnetwork which can group together collections of devices that are connected to separate physical LANs.
VLANs allow network administrators to partition a single switched network in order to keep network applications separate despite being connected to the same physical network, without requiring new cabling or major changes in the current network infrastructure.
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Network redundancy is a method to ensure network availability, providing failover when a device or network path fails or becomes unavailable.
Redundancy is usually achieved by installing additional or alternative network devices, communication media or equipment within the network infrastructure

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PRP & iGrid
Redundancy has been a key element in many iGrid projects, which is why all of our devices support various redundant schemes and networks, including PRP, HSR and RSTP.
iRTU/iGW‑S#01 devices can acts as PRP DANP and RedBox (Redundancy Box)

iRTU – With I/Os for Direct Data Acquisition
Compact and scalable bay controller which can act as IEC 61850 client or server, featuring configurable I/O boards for direct data acquisition, high-precision timestamping and an optional Ethernet switch for additional Ethernet ports.

iGW‑S Substation Gateway
Powerful and reliable substation gateway, able to run either in standalone or redundant modes, with an embedded Ethernet switch (4 ports) and IEC 61850 client and server capabilities.
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