IEC 62439-3:2012

Industrial communication networks - High availability automation networks - Part 3: Parallel Redundancy Protocol (PRP) and High-availability Seamless Redundancy (HSR)

IEC 62439-3:2012 specifies two redundancy protocols designed to provide seamless recovery in case of single failure of an inter-bridge link or bridge in the network, which are based on the same scheme: duplication of the LAN, resp. duplication of the transmitted information. This second edition cancels and replaces the first edition published in 2010. It constitutes a technical revision. The main changes with respect to the previous edition are:
- specification of the interconnection of PRP and HSR networks;
- introduction of a suffix for PRP frames;
- clarification and modification of specifications to ensure interoperability;
- slackening of the specifications to allow different implementations;
- consideration of clock synchronization according to IEC 61588;
- introduction of test modes to simplify testing and maintenance.
This publication is to be read in conjunction with IEC 62439-1:2010.

Réseaux industriels de communication - Réseaux d'automatisme à haute disponibilité - Partie 3: Protocole de redondance parallèle (PRP) et redondance transparente de haute disponibilité (HSR)

La CEI 62439-3:2012 spécifie deux protocoles de redondance conçus pour assurer une reprise transparente en cas de défaillance unique d'une maille inter-étage ou d'un pont au sein du réseau, qui reposent sur le même schéma: duplication du réseau local, duplication des informations transmises. Cette deuxième édition annule et remplace la première édition parue en 2010. Elle constitue une révision technique. Les principales modifications apportées à l'édition précédente sont:
- spécification de l'interconnexion des réseaux PRP et HSR;
- introduction d'un suffixe pour les trames PRP;
- clarification et modification des spécifications pour garantir l'interopérabilité;
- assouplissement des spécifications pour permettre des mises en oeuvre différentes;
- prise en compte de la synchronisation des horloges conformément à la CEI 61588;
- introduction de modes d'essai visant à simplifier les essais et la maintenance.
Cette publication doit être lue conjointement avec la CEI 62439-1:2010.

General Information

• Status: Published
• Publication Date: 04-Jul-2012
• ICS: 25.040.40 - Industrial process measurement and control
• ICS: 35.100.05 - Multilayer applications
• Technical Committee: SC 65C - Industrial networks
• Drafting Committee: WG 15 - TC 65/SC 65C/WG 15
• Current Stage: DELPUB - Deleted Publication
• Start Date: 31-Mar-2016
• Completion Date: 26-Oct-2025

Relations

• Revised: IEC 62439-3:2016 - Industrial communication networks - High availability automation networks - Part 3: Parallel Redundancy Protocol (PRP) and High-availability Seamless Redundancy (HSR)
• Effective Date: 05-Sep-2023
• Revises: IEC 62439-3:2010 - Industrial communication networks - High availability automation networks - Part 3: Parellel Redundancy Protocol (PRP) and High-availability Seamless Redundancy (HSR)
• Effective Date: 05-Sep-2023

Overview

IEC 62439-3:2012 is an international standard developed by the International Electrotechnical Commission (IEC) that specifies protocols for high availability automation networks in industrial communication systems. Specifically, it addresses the Parallel Redundancy Protocol (PRP) and High-availability Seamless Redundancy (HSR), two redundancy protocols designed to ensure seamless network recovery in case of single failures within the network infrastructure. This second edition updates and expands the 2010 release with technical revisions to improve interoperability, flexibility of implementation, and support for clock synchronization.

The standard is essential for industries requiring ultra-reliable, uninterrupted communication in automation networks, such as power utilities, manufacturing, and process control sectors. By duplicating network LANs or transmitted data, PRP and HSR provide zero-time recovery and eliminate packet loss, critical for time-sensitive industrial operations.

Key Topics

• Redundancy Protocols: The standard covers two main protocols-PRP, which uses two independent LANs in parallel, and HSR, which relies on a ring topology to duplicate frames across the network.

• Seamless Recovery: Both protocols ensure zero recovery time following a single failure of an inter-bridge link or a network bridge, maintaining continuous communication without loss of data.

• Network Topologies:

  • PRP supports parallel LAN configurations with bus, linear, or ring topologies.
  • HSR primarily focuses on ring topologies but also supports complex meshed and interconnected ring networks.

• Interoperability Enhancements: New specifications clarify interconnection mechanisms between PRP and HSR networks, allowing combined implementations that leverage the strengths of both protocols.

• Protocol Mechanics:

  • PRP introduces a suffix in frame formats for easy identification and duplicate frame management.
  • HSR utilizes a specific frame format to handle duplicated frames across ring nodes, including support for VLAN tagging.

• Clock Synchronization: The standard incorporates provisions for time synchronization consistent with IEC 61588 (Precision Time Protocol), ensuring synchronization crucial for deterministic and real-time communication.

• Network Supervision and Management: Includes detailed specifications for supervision frames, management information bases (MIBs), and test modes that aid operational monitoring, diagnostics, and maintenance.

• Flexibility Improvements: The updated standard relaxes certain implementation constraints to allow varied vendor solutions while maintaining compatibility and interoperability.

Applications

IEC 62439-3:2012 is widely applied in industries where high availability and zero downtime communication networks are mandatory:

• Electric Power Generation and Distribution: Ensures reliable communication in substation automation and smart grid systems where continuous operation is critical.

• Industrial Automation: Facilitates robust, redundant communication for factory automation systems, reducing production losses due to network faults.

• Process Industries: Enables seamless redundancy for process control networks in oil & gas, chemical plants, and pharmaceuticals, where interruption can lead to critical safety risks.

• Transportation: Used in railway signaling and control systems demanding high resilience and rapid failure recovery.

Implementing PRP or HSR in these environments helps achieve fault tolerance, network resilience, and real-time deterministic communication that underpin modern industrial Ethernet solutions.

Related Standards

To fully understand and implement IEC 62439-3:2012, familiarity with related IEC and IEEE standards is recommended:

• IEC 62439-1:2010: Provides general requirements and architectural considerations for high availability automation networks.

• IEC 61588: Specifies clock synchronization protocols based on Precision Time Protocol (PTP), which IEC 62439-3 aligns with for time-sensitive networking.

• IEEE 802.1Q: Pertains to VLAN tagging, which is supported in PRP and HSR frame formats for traffic segregation.

• IEC 61850 Series: Although focusing on power utility automation, it often incorporates IEC 62439-3 protocols to ensure network redundancy.

Implementers and network designers should evaluate these complementary standards to design integrated, scalable, and interoperable high-availability industrial communication networks.

---

By following IEC 62439-3:2012, organizations can deploy robust redundancy protocols that guarantee continuous network operation, enhanced fault tolerance, and faster recovery mechanisms, ultimately increasing the reliability and safety of critical industrial automation communications.

Standards Content (Sample)

IEC 62439-3 ®
Edition 2.0 2012-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Industrial communication networks – High availability automation networks –
Part 3: Parallel Redundancy Protocol (PRP) and High-availability Seamless
Redundancy (HSR)
Réseaux industriels de communication – Réseaux d'automatisme à haute
disponibilité –
Partie 3: Protocole de redondance parallèle (PRP) et redondance transparente de
haute disponibilité (HSR)
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IEC 62439-3 ®
Edition 2.0 2012-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Industrial communication networks – High availability automation networks –

Part 3: Parallel Redundancy Protocol (PRP) and High-availability Seamless

Redundancy (HSR)
Réseaux industriels de communication – Réseaux d'automatisme à haute

disponibilité –
Partie 3: Protocole de redondance parallèle (PRP) et redondance transparente de

haute disponibilité (HSR)
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX XC
ICS 25.040; 35.040 ISBN 978-2-83220-160-2

– 2 – 62439-3 © IEC:2012
CONTENTS
FOREWORD . 5
0 INTRODUCTION . 7
0.1 General . 7
0.2 Changes with respect to the previous edition . 7
0.3 Patent declaration . 8
1 Scope . 9
2 Normative references . 9
3 Terms, definitions, abbreviations, acronyms, and conventions . 10
3.1 Terms and definitions . 10
3.2 Abbreviations and acronyms . 10
3.3 Conventions . 10
4 Parallel Redundancy Protocol (PRP) . 10
4.1 PRP principle of operation . 10
4.1.1 PRP network topology . 10
4.1.2 PRP LANs with linear or bus topology . 11
4.1.3 PRP LANs with ring topology . 12
4.1.4 DANP node structure . 12
4.1.5 PRP attachment of singly attached nodes . 13
4.1.6 Compatibility between singly and doubly attached nodes . 14
4.1.7 Network management . 14
4.1.8 Implication on configuration . 14
4.1.9 Transition to non-redundant networks . 14
4.1.10 Duplicate handling . 15
4.1.11 Network supervision . 19
4.1.12 Redundancy management interface . 19
4.2 PRP protocol specifications . 19
4.2.1 Installation, configuration and repair guidelines . 19
4.2.2 MAC addresses . 20
4.2.3 Multicast MAC addresses . 20
4.2.4 IP addresses . 20
4.2.5 Nodes . 20
4.2.6 Duplicate Accept mode (testing only) . 21
4.2.7 Duplicate Discard mode . 21
4.3 PRP_Supervision frame . 24
4.3.1 Supervision frame for DANP . 24
4.3.2 PRP_Supervision frame contents . 27
4.3.3 PRP_Supervision frame for RedBox . 27
4.3.4 Reception of a PRP_Supervision frame and NodesTable . 27
4.4 Bridging node . 28
4.5 Constants . 28
4.6 PRP service specification . 28
5 High-availability Seamless Redundancy (HSR) . 28
5.1 HSR objectives . 28
5.2 HSR principle of operation . 29
5.2.1 Basic operation with a ring topology . 29
5.2.2 DANH node structure . 30

62439-3 © IEC:2012 – 3 –
5.2.3 Topology . 31
5.2.4 RedBox structure . 39
5.3 HSR node specifications . 41
5.3.1 HSR operation . 41
5.3.2 DANH receiving from its link layer interface . 41
5.3.3 DANH receiving from an HSR port . 42
5.3.4 DANH forwarding rules . 43
5.3.5 CoS . 43
5.3.6 Clock synchronization . 43
5.3.7 Deterministic medium access . 44
5.4 HSR RedBox specifications . 44
5.4.1 RedBox properties . 44
5.4.2 RedBox receiving from interlink . 44
5.4.3 RedBox forwarding on the ring. 46
5.4.4 RedBox receiving from an HSR port . 46
5.4.5 RedBox receiving from its link layer interface . 47
5.4.6 Redbox ProxyNodeTable handling . 47
5.4.7 RedBox CoS . 48
5.4.8 RedBox clock synchronization . 48
5.4.9 RedBox medium access . 48
5.5 QuadBox specification . 48
5.6 Duplicate Discard method. 48
5.7 Frame format for HSR . 48
5.7.1 Frame format for all frames . 48
5.7.2 HSR_Supervision frame . 50
5.8 Constants . 52
5.9 HSR service specification . 53
6 Protocol Implementation Conformance Statement (PICS) . 54
7 PRP/HSR Management Information Base (MIB) . 55
Annex A (normative) Use of IEC 61588 and IEEE C37.238 for IEC 62439-3 . 69
Annex B (informative) Deterministic medium access in HSR . 83
Bibliography . 84

Figure 1 – PRP example of general redundant network . 11
Figure 2 – PRP example of redundant network as two LANs (bus topology) . 12
Figure 3 – PRP example of redundant ring with SANs and DANPs . 12
Figure 4 – PRP with two DANPs communicating . 13
Figure 5 – PRP RedBox, transition from single to double LAN . 15
Figure 6 – PRP frame extended by an RCT. 16
Figure 7 – PRP VLAN-tagged frame extended by an RCT . 17
Figure 8 – PRP padded frame closed by an RCT . 17
Figure 9 – Duplicate Discard algorithm boundaries . 18
Figure 10 – HSR example of ring configuration for multicast traffic . 29
Figure 11 – HSR example of ring configuration for unicast traffic . 30
Figure 12 – HSR structure of a DANH . 31
Figure 13 – HSR example of topology using two independent networks . 32

– 4 – 62439-3 © IEC:2012
Figure 14 – HSR example of peer coupling of two rings . 33
Figure 15 – HSR example of connected rings . 34
Figure 16 – HSR example of coupling two redundant PRP LANs to a ring . 35
Figure 17 – HSR example of coupling from a ring node to redundant PRP LANs . 36
Figure 18 – HSR example of coupling from a ring to two PRP LANs . 37
Figure 19 – HSR example of coupling three rings to one PRP LAN . 38
Figure 20 – HSR example of meshed topology . 39
Figure 21 – HSR structure of a RedBox . 40
Figure 22 – HSR frame without a VLAN tag . 49
Figure 23 – HSR frame with VLAN tag . 49
Figure 24 – HSR node with management counters . 53
Figure 25 – HSR RedBox with management counters . 54
Figure A.1 – PTP one-step clock synchronization and delay measurement . 71
Figure A.2 – PTP two-step clock synchronization and delay measurement . 72
Figure A.3 – Two-step and one-step transparent clocks translator . 73
Figure A.4 – Two-step to one-step translation . 73
Figure A.5 – Connection of a Grandmaster Clock to an Ordinary Clock over PRP . 74
Figure A.6 – HSR with one GMC . 76
Figure A.7 – PTP messages sent and received by an HSR node (one-step) . 77
Figure A.8 – PTP messages sent and received by an HSR node (two-step) . 78
Figure A.9 – Attachment of a GMC to an HSR ring through a RedBox . 80
Figure A.10 – PRP to HSR coupling by Transparent Clocks . 81
Figure A.11 – PRP to HSR coupling by BCs . 82

Table 1 – NodesTable attributes . 22
Table 2 – PRP_Supervision frame with no VLAN tag. 25
Table 3 – PRP_Supervision frame with (optional) VLAN tag . 26
Table 4 – PRP constants . 28
Table 5 – HSR_Supervision frame with no VLAN tag . 50
Table 6 – HSR_Supervision frame with optional VLAN tag . 51
Table 7 – HSR Constants . 52

62439-3 © IEC:2012 – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
INDUSTRIAL COMMUNICATION NETWORKS –
HIGH AVAILABILITY AUTOMATION NETWORKS –

Part 3: Parallel Redundancy Protocol (PRP) and
High-availability Seamless Redundancy (HSR)

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 62439-3 has been prepared by subcommittee 65C: Industrial
networks, of IEC technical committee 65: Industrial-process measurement, control and
automation.
This second edition cancels and replaces the first edition published in 2010. This edition
constitutes a technical revision. The main changes with respect to the previous edition are
listed below:
– specification of the interconnection of PRP and HSR networks;
– introduction of a suffix for PRP frames;
– clarification and modification of specifications to ensure interoperability;
– slackening of the specifications to allow different implementations;
– consideration of clock synchronization according to IEC 61588;
– introduction of test modes to simplify testing and maintenance.

– 6 – 62439-3 © IEC:2012
The text of this standard is based on the following documents:
FDIS Report on voting
65C/687/FDIS 65C/705/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This International Standard is to be read in conjunction with IEC 62439-1:2011.
A list of the IEC 62439 series can be found, under the general title Industrial communication
networks – High availability automation networks, on the IEC website.
This publication has been drafted in accordance with ISO/IEC Directives, Part 2.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
62439-3 © IEC:2012 – 7 –
0 INTRODUCTION
0.1 General
IEC 62439-3 standard belongs to IEC 62439 series, Industrial communication networks – High
availability automation networks, specifying the HSR and PRP redundancy protocols, and was
adopted by TC57 WG10 as the redundancy method for demanding substation automation
networks based on IEC 61850 series, introducing new requirements.
0.2 Changes with respect to the previous edition
The major changes with respect to IEC 62439-3:2010 are listed below.
Aligning the sequence number between PRP and HSR, to enable coupling of HSR and PRP
networks and simplify the implementation of dual-mode nodes in hardware. At the same time,
introduce a suffix in the PRP Redundancy Control Trailer to allow better identification, future
extensions and coexistence with other protocols that also happen to use a trailer. This change
is not backwards-compatible, so means are provided to identify the version and ensure that
the networks are homogeneous.
Removing all implementation restrictions on the Duplicate Discard algorithm (especially
references to the drop window algorithm and references to connection orientation) since other
methods such as hash tables can be used.
Removing the purging of the duplicate table. Replace this specific method by requiring that
any Duplicate Discard algorithm provides a mechanism to remove old entries, thus ensuring
that a node can properly reboot.
Making node tables optional for simple nodes to simplify hardware implementation.
Suppression of explicit mention of the HSR-PRP mode (PRP with HSR Tags), but allow it
through the Mode N (no forwarding).
Introducing Mode T (forward through) to allow maintenance laptops to configure an open ring
when attached to one end and Mode M (mixed) to allow forwarding of non-HSR-tagged frames
in a closed ring.
Recommending the position of connectors, rather than impose it.
Defining the behaviour of an HSR node when non-HSR frames are encountered without
requiring the recording of the source addresses and specify how IEEE 802.1D:2004, Table 7-
10 frames are treated.
Prefixing the supervision frames on HSR by an HSR tag to simplify the hardware
implementation and introduce a unique EtherType for HSR to simplify processing.
Changing the rule for the RedBox to allow more than one PRP network to be connected to an
HSR ring, and introduce an identifier per RedBox pair.
Specifying tagging of IEC 61588 frames to follow IEEE C37.238 recommendations (informal).
Suppressing MAC address substitution.
Adapting the MIB to above changes.

– 8 – 62439-3 © IEC:2012
0.3 Patent declaration
The International Electrotechnical Commission (IEC) draws attention to the fact that it is
claimed that compliance with this document may involve the use of a patent concerning
Filtering of redundant frames in a network node given in 5.2.3.3.
IEC takes no position concerning the evidence, validity and scope of this patent right.
The holder of this patent right has assured the IEC that he/she is willing to negotiate licences
under reasonable and non-discriminatory terms and conditions with applicants throughout the
world. In this respect, the statement of the holder of this patent right is registered with IEC.
Information may be obtained from:
Siemens Aktiengesellschaft
80333 München, Germany
The International Electrotechnical Commission (IEC) draws attention to the fact that it is
claimed that compliance with this document may involve the use of a patent concerning
Reception of redundant and non-redundant frames (ABB Schweiz AG – WO 2006/053459 A1,
EP 1825657, US 20070223533, CN 101057483) given in 4.2.7, concerning Identifying
improper cabling of devices (ABB Schweiz AG – EP 2 015 501 A1) given in 4.3, concerning
Critical device with increased availability (ABB Schweiz AG – EP 2 090 950 A1) given in 4.4,
concerning Ring coupling nodes for high availability networks (ABB Schweiz AG – WO
2010/010120 A1) given in 5.2.3.
IEC takes no position concerning the evidence, validity and scope of this patent right.
The holder of this patent right has assured the IEC that he/she is willing to negotiate licences
under reasonable and non-discriminatory terms and conditions with applicants throughout the
world. In this respect, the statement of the holder of this patent right is registered with IEC.
Information may be obtained from:
ABB Schweiz AG
Brown Boveri Strasse 6
5400 Baden, Switzerland
Attention is drawn to the possibility that some of the elements of this document may be the
subject of patent rights other than those identified above. IEC shall not be held responsible for
identifying any or all such patent rights.
ISO (www.iso.org/patents) and IEC (http://patents.iec.ch) maintain on-line data bases of
patents relevant to their standards. Users are encouraged to consult the data bases for the
most up to date information concerning patents.

62439-3 © IEC:2012 – 9 –
INDUSTRIAL COMMUNICATION NETWORKS –
HIGH AVAILABILITY AUTOMATION NETWORKS –

Part 3: Parallel Redundancy Protocol (PRP) and
High-availability Seamless Redundancy (HSR)

1 Scope
The IEC 62439 series is applicable to high-availability automation networks based on the
ISO/IEC 8802-3 (Ethernet) technology.
This part of the IEC 62439 series specifies two redundancy protocols designed to provide
seamless recovery in case of single failure of an inter-bridge link or bridge in the network,
which are based on the same scheme: duplication of the LAN, resp. duplication of the
transmitted information.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60050-191, International Electrotechnical Vocabulary – Chapter 191 : Dependability and
quality of service
IEC 61588, Precision clock synchronization protocol for networked measurement and control
systems
IEC 62439-1, Industrial communication networks – High availability automation networks –
Part 1: General concepts and calculation methods
IEC 62439-2, Industrial communication networks – High availability automation networks –
Part 2: Media Redundancy Protocol (MRP)
IEC 62439-6, Industrial communication networks – High availability automation networks –
Part 6: Distributed Redundancy Protocol (DRP)
IEC 62439-7, Industrial communication networks – High availability automation networks –
Part 7: Ring-based Redundancy Protocol (RRP)
ISO/IEC 8802-3:2000, Information technology – Telecommunications and information
exchange between systems – Local and metropolitan area networks – Specific requirements –
Part 3: Carrier sense multiple access with collision detection (CSMA/CD) access method and
physical layer specifications
IEEE 802.1D:2004, IEEE Standard for Local and Metropolitan Area Networks – Media Access
Control (MAC) Bridges
IEEE 802.1Q:2011, IEEE Standard for Local and Metropolitan Area Networks – Media Access
Control (MAC) Bridges and Virtual Bridge Local Area Network

– 10 – 62439-3 © IEC:2012
3 Terms, definitions, abbreviations, acronyms, and conventions
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60050-191, as well
as in IEC 62439-1, as well as the following, apply.
3.1.1
interlink
link that connects two network hierarchies
3.1.2
RedBox
device attaching single attached nodes to a redundant network
3.1.3
QuadBox
quadruple port device connecting two peer HSR rings, which behaves as an HSR node in
each ring and is able to filter the traffic and forward it from ring to ring
3.1.4
HSR frame
frame that carries as EtherType the HSR_ethertype
3.1.5
switching logic
hardware that transmits a frame from one port to another port, possibly providing cut-through
Note 1 to entry: In this document, the words “bridge” respectively “bridging” are synonymous to the words
“switch”, respectively “switching” when they apply to layer 2 connectivity.
3.2 Abbreviations and acronyms
For the purposes of this document, the abbreviations and acronyms given in IEC 62439-1, as
well as the acronyms below, apply.
CoS Class of Service (layer 2 quality of service)
DANH Double attached node implementing HSR
DANP Double attached node implementing PRP
RCT Redundancy Check Trailer
VDAN Virtual Doubly Attached Node (SAN as visible through a RedBox)
3.3 Conventions
This part of the IEC 62439 series follows the conventions defined in IEC 62439-1.
4 Parallel Redundancy Protocol (PRP)
4.1 PRP principle of operation
4.1.1 PRP network topology
This redundancy protocol implements redundancy in the nodes rather than in the network,
using doubly attached nodes obeying to PRP (DANPs).

62439-3 © IEC:2012 – 11 –
A DANP is attached to two independent Local Area Networks (LANs) of similar topology,
named LAN_A and LAN_B, which operate in parallel. A source DANP sends the same frame
over both LANs and a destination DANP receives it from both LANs within a certain time,
consumes the first frame and discards the duplicate.
Figure 1 shows a redundant network consisting of two LANs, each of which can have any
topology, e.g. tree, ring or meshed.
source
DANP B-frame DANP
SAN
A-frame
A1
bridge bridge
bridged local area bridged local area
network (ring) network (tree)
LAN_A LAN_B
bridge
bridge bridge bridge
SAN
SAN SAN
A2
RedBox
B1 B2
A-frame
B-frame
SAN SAN
DANP DANP DANP
IEC  1128/12
R1 R2
destination
Figure 1 – PRP example of general redundant network
The two LANs are identical in protocol at the MAC-LLC level, but they can differ in
performance and topology. Transmission delays may also be different, especially if one of the
networks reconfigures itself, e.g. using RSTP, to overcome an internal failure.
The two LANs follow configuration rules that allow the network management protocols such as
Address Resolution Protocol (ARP) and Simple Network Management Protocol (SNMP) to
operate correctly.
The two LANs have no connection between them and are assumed to be fail-independent.
Redundancy can be defeated by single points of failure, such as a common power supply or a
direct connection whose failure brings both networks down. Installation guidelines in this
document provide guidance to the installer to achieve fail-independence.
4.1.2 PRP LANs with linear or bus topology
As an example of a simpler topology, Figure 2 draws a PRP network as two LANs in linear
topology, which may also be a bus topology.

– 12 – 62439-3 © IEC:2012
DADANPNP DADANPNP DADANPNP DADANPNP DADANPNP DADANPNP
LAN_A
LAN_B
IEC  1129/12
Figure 2 – PRP example of redundant network as two LANs (bus topology)
4.1.3 PRP LANs with ring topology
The two LANs can have a ring topology, as Figure 3 shows.
DADANPNP DADANPNP
bridge
bridge
bridge bridge bridge
bridge bridge bridge
DANP
DANP DANP
. . .
RedBox
SAN
DANP DANP
DANP SAN SAN
IEC  1130/12
Figure 3 – PRP example of redundant ring with SANs and DANPs
NOTE In this case ring redundancy is provided by a different mechanism, for instance as defined in IEEE 802.1D
or in other IEC 62439 protocols, i.e. MRP.
4.1.4 DANP node structure
Each node has two ports that operate in parallel and that are attached to the same upper
layers of the communication stack through the Link Redundancy Entity (LRE), as Figure 4
shows.
62439-3 © IEC:2012 – 13 –
DANP 1 DANP 2
UDP TCP
UDP TCP
hard real-time hard real-time
upper layers
stack stack
network layer network layer
same data
Link Redundancy Entity Link Redundancy Entity
link layer
interface
port A port B port A port B
network
adapters
Tx Rx Tx Rx
Tx Rx Tx Rx
transceivers
LAN_A
LAN_B
IEC  1131/12
Figure 4 – PRP with two DANPs communicating
For the basic communication, the LRE presents toward its upper layers the same interface as
a non-redundant network adapter, so the upper layers are unaware of redundancy.
The LRE has two tasks: handling of duplicates and management of redundancy.
When receiving a frame from the node’s upper layers, the LRE appends to the frame a
Redundancy Check Trailer (RCT) containing a sequence number and sends the frame through
both its ports at nearly the same time. The two frames are nearly identical except for the LAN
identifier (and the checksum).
The two frames transit through the two LANs with different delays, ideally they arrive at nearly
the same time at the destination node.
When receiving frames from the network, the LRE forwards the first received frame of a pair
to its node’s upper layers and discards the duplicate frame (if it arrives). It removes the RCT if
required.
For management of redundancy and checking of the presence of other DANPs, an LRE
periodically sends PRP_Supervision frames and can evaluate the PRP_Supervision frames
sent by other DANPs.
4.1.5 PRP attachment of singly attached nodes
Singly attached nodes (SANs) can be attached in two ways:
SANs can be attached directly to one LAN only. Such SANs can only communicate with other
SANs on the same LAN. For instance, in Figure 1, SAN A1 can communicate with SAN A2,
but not with SAN B1 or SAN B2. SANs can communicate (not redundantly) with all DANPs.
SANs can be attached over a RedBox (redundancy box) to both LANs, as Figure 1 shows for
SAN R1 and SAN R2 (see also 4.1.9). Such SANs can communicate with all DANP and SANs,
for instance SAN A1 and SAN R1 can communicate.
NOTE SANs are not aware of PRP; they can be off-the-shelf computers or printers.

– 14 – 62439-3 © IEC:2012
In some applications, only availability-critical devices need a double attachment, for instance
the operator workplaces, while the majority of the devices are SANs. Taking advantage of the
basic infrastructure of PRP, a DANP can be attached to two different bridges of the same LAN
(e.g. a ring) and use protocols different from PRP to reconfigure the network in case of failure.
The DANP then behaves as a bridge according to IEEE 802.1D. For instance, the bridge
element may implement the RSTP protocol or a subset of RSTP where there is no forwarding
of traffic between the ports. These abilities are optional and not detailed in this International
Standard. The supported mode is specified in the PICS (see 5.9).
4.1.6 Compatibility between singly and doubly attached nodes
Singly attached nodes (SAN), for instance maintenance laptops or printers that belong to one
LAN, can be connected to any LAN. Bridges are always SANs. A SAN connected to one LAN
cannot communicate directly to a SAN connected to the other LAN. These SANs are not
aware of PRP redundancy, but DANPs generate a traffic that these SANs understand. The
condition is however that the SANs ignore the RCT in the frames, which should be the case
since a SAN cannot distinguish the RCT from the Ethernet padding. Conversely, DANPs
understand the traffic generated by SANs, since these do not append a RCT. They only
forward one frame to their upper layers since the SAN traffic uses one LAN only.
4.1.7 Network management
A node has the same MAC address on both ports, and only one set of IP addresses assigned
to that address. This makes redundancy transparent to the upper layers and especially allows
the Address Resolution Protocol (ARP) to work as with a SAN. Bridges in a LAN are not
doubly attached devices, and therefore all managed bridges have different IP (and MAC)
addresses. A network management tool is preferably a DANP and can access nodes and
bridges in both LANs.
4.1.8 Implication on configuration
Since the same frame can come from the two ports with significant time difference, a frame
could be wrongly rejected as duplicate in exceptional situations, so mechanisms exist to age
out frames, which is seldom cases cause both duplicates to be accepted – a situation
applications tolerate. Possibly the heaviest traffic comes from sampled data, whose period
should be chosen considering the worst-case difference in propagation delays.
4.1.9 Transition to non-redundant networks
The mechanism of duplicate rejection can be implemented by a RedBox that does the
transition between a SAN and the doubled LANs, as Figure 5 shows. The RedBox mimics the
SANs connected behind it (called VDAN or virtual DANs) and multicasts supervision frames
on their behalf. The RedBox is itself a DANP and has its own IP address for management
purposes, but it may also perform application functions.

62439-3 © IEC:2012 – 15 –
bridge
non-redundant network
SAN SAN SAN
S1 S2 S3
singly attached nodes
local
application
Tx Rx
port C
TCP/IP
SNMP
RedBox
LRE
port A port B
Tx Rx Tx Rx
transceivers
LAN_A
LAN_B
IEC  1132/12
Figure 5 – PRP RedBox, transition from single to double LAN
4.1.10 Duplicate handling
4.1.10.1 Duplicate Accept mode (testing only)
The “Duplicate Accept” mode is used for testing purposes, to verify that duplicates are indeed
discarded by the link layer and not by higher layer protocols.
In this mode, a sender is configured to send both frames without RCT.
In this mode, a receiver is configured to accept both frames and forward both of them (if both
arrive) to its upper layers.
A node indicates its mode (Duplicate Accept /Duplicate Discard) in its supervision frames.
NOTE Upper layers also discard duplicates, but less effectively than the link layer.
4.1.10.2 Duplicate Discard mode
4.1.10.2.1 Principle
To allow the receivers to detect duplicates, the sender LRE appends a six-octet field that
contains a sequence number, the Redundancy Control Trailer (RCT) to both frames it sends,
as Figure 6 shows.
The receiver LRE uses the sequence number of the RCT and the source MAC address to
detect duplicates. It forwards only the first frame of a pair to its upper layers. To offload the

– 16 – 62439-3 © IEC:2012
application processor, the LRE can be implemented with an independent pre-processor, an
intelligent Ethernet controller or in hardware.
Keeping track of duplicates allows at the same time to improve the redundancy supervision.
As an option, the receiver may remove the RCT before forwarding the (first) frame to its upper
layer.
The RCT consists of the following fields:
• 16-bit sequence number (SeqNr);
• 4-bit LAN identifier (LanId);
• 12 bit frame size (LSDUsize);
• 16-bit suffix (PRPsuffix).
octet position 0 6 12 14
LSDU PRP
preamble destination source LT LSDU SeqNr FCS
size suffix
time
frame without redundancy control Redundancy Control Trailer
IEC  1133/12
Figure 6 – PRP frame extended by an RCT
The LRE sends both (nearly identical) frames over both LANs.
The receiving LRE can then detect duplicates based on the RCT.
NOTE 1 This method considers that SANs also exist on the network, and that frames sent by SANs could be
wrongly rejected as duplicates because they happen to have a trailing field with the correct PRPsuffix, same
sequence number and th
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