Redundancy Protocols; Parallel Redundancy Protocol (Prp); Prp Networks; Network Elements - GE MiCOM P747 Agile Technical Manual

Chapter 8 - Redundant Ethernet
4

REDUNDANCY PROTOCOLS

The following redundancy protocols are available:
● PRP (Parallel Redundancy Protocol)
● RSTP (Rapid Spanning Tree Protocol)
● SHP (Self-Healing Protocol)
● DHP (Dual Homing Protocol)
The protocol must be selected at the time of ordering.
4.1

PARALLEL REDUNDANCY PROTOCOL (PRP)

Power system companies have traditionally used proprietary protocols for redundant communications. This is
because standardized protocols could not meet the requirements for real-time systems. Even a short loss of
connectivity may result in loss of functionality.
However, Parallel Redundancy Protocol (PRP) uses the IEC 62439 standard in Dual Homing Star Topology
networks, designed for IEDs from different manufacturers to operate with each other in a substation
redundant-Ethernet network. PRP provides bumpless redundancy for real-time systems and is the standard
for double Star-topology networks in substations.
4.1.1

PRP NETWORKS

Redundant networks usually rely on the network's ability to reconfigure if there is a failure. However, PRP
uses two independent networks in parallel.
PRP implements the redundancy functions in the end nodes rather than in network elements. This is one
major difference to RSTP. An end node is attached to two similar LANs of any topology which operate in
parallel.
The sending node replicates each frame and transmits them over both networks. The receiving node
processes the frame that arrives first and discards the duplicate. Therefore there is no distinction between
the working and backup path. The receiving node checks that all frames arrive in sequence and that frames
are correctly received on both ports.
The PRP layer manages this replicate and discard function, and hides the two networks from the upper
layers. This scheme works without reconfiguration and switchover, so it stays available ensuring no data
loss.
There should be no common point of failure between the two LANs. Therefore they are not powered by the
same source and cannot be connected directly together. They are identical in protocol at the MAC level but
may differ in performance and topology. Both LANs must be on the same subnet so all IP addresses must be
unique.
4.1.2

NETWORK ELEMENTS

A PRP compatible device has two ports that operate in parallel. Each port is connected to a separate LAN. In
the IEC 62439 standard, these devices are called DANP (Doubly Attached Node running PRP). A DAN has
two ports, one MAC address and one IP address.
A Single Attached Node (SAN) is a non-critical node attached to only one LAN. SANs that need to
communicate with each other must be on the same LAN.
The following diagram shows an example of a PRP network. The Doubly Attached Nodes DANP 1 and
DANP 2 have full node redundancy. The Singly Attached Nodes SAN 1 and SAN 4 do not have any
redundancy. Singly attached nodes can be connected to both LANs using a Redundancy Box (RedBox). The
RedBox converts a singly attached node into a doubly attached node. Devices such as PCs with one
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