What is PRP (Parallel Redundancy Protocol) and how does it work?

PRP (Parallel Redundancy Protocol), defined in IEC 62439-3, provides zero-recovery-time redundancy for critical networks by transmitting frames simultaneously over two independent networks.

How PRP Works

1. Dual-Attached Node (DAN) has two network interfaces
2. Sender transmits identical frame on both networks simultaneously
3. Receiver accepts first arriving frame, discards duplicate
4. No wait or switchover on failure — no recovery time

Key Components

• DAN (Dual Attached Node): Device with PRP support
• SAN (Single Attached Node): Standard device
• RedBox: Converts SANs to appear as DANs
• LAN A and LAN B: Independent networks

Frame Structure

• PRP adds RCT (Redundancy Control Trailer) to Ethernet frame
• RCT contains sequence number and network ID
• Duplicate detection based on sequence number

Comparison with STP/RSTP

Advantages

• True zero-recovery-time
• Any topology supported
• No spanning tree recalculation
• Independent networks can differ in speed/vendor
• Better for time-critical protection

Disadvantages

• Requires duplicate infrastructure
• Higher cost
• Duplicate bandwidth consumption
• Not all devices support PRP natively

Where PRP is Used

• Substation process bus (IEC 61850-9-2)
• Critical protection networks
• Nuclear power plants
• Railway safety systems
• High-availability industrial control

PRP vs HSR

• PRP: Two independent networks
• HSR: Single ring with duplicate frames
• Both provide zero recovery time
• PRP more flexible, HSR more efficient with fiber