What is PRP?

• 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

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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