High-availability Seamless Redundancy Protocol
The concept of HSR comprises the following network elements and options (see figure)
- A node integrated directly in the ring is denoted as a Double Attached Node implementing HSR (DANH). It is a device with an integrated three port HSR switch (two external and an internal port). A frame to be sent is duplicated and then transmitted in both directions of the ring. Intermediate nodes forward the frames. Unicast frames are removed from the ring by the receiver, multicast and broadcast frames by the sender. The receiver processes the first frame received and discards duplicates.
- HSR frames are labelled by a header extension which allows to identify them and facilitates the detection of duplicates.
- Non-HSR-capable devices can be integrated into the network by means of a Redundancy Box (RedBox). The RedBox provides the HSR functions on behalf of the nodes connected to it. Thanks to this proxy functions nodes beyond a RedBox appear like double attached nodes an are therefore called Virtual Doubly Attached Nodes (VDAN).
- HSR rings may be interconnected by Quadboxes. A redundant coupling requires two Quadboxes.
- Networks running the Parallel Redundancy Protocol (PRP) can be connected to HSR networks by means of RedBoxes. A PRP end node is denoted as Double Attached Node implementing PRP (DANP).