What is HSR (High-availability Seamless Redundancy)?

HSR (High-availability Seamless Redundancy), part of IEC 62439-3, provides zero-recovery-time redundancy using a ring topology with duplicate frame transmission in opposite directions.

How HSR Works

1. Nodes connect in a ring with two ports
2. Sender injects duplicate frame in both directions (clockwise and counter-clockwise)
3. Frames travel around ring until reaching destination
4. Receiver accepts first frame, discards duplicate
5. If one path is broken, other still delivers frames

HSR Frame

• Standard Ethernet + HSR tag (6 bytes)
• Contains sequence number and lane info
• All nodes forward frames toward destination

Key Concepts

• DANH (Doubly Attached Node HSR): HSR-capable device
• SAN (Single Attached Node): Standard device (needs QuadBox or RedBox)
• QuadBox: Connects two HSR rings
• RedBox HSR: Attaches non-HSR devices

Advantages Over PRP

• Single ring instead of two networks (less cabling)
• Lower infrastructure cost
• Zero recovery time preserved
• Efficient use of fiber

Disadvantages

• All nodes must be HSR-capable (or use RedBox)
• Ring size limits (typical max 50 nodes)
• Duplicate traffic uses ring bandwidth twice
• More complex than simple ring protocols