Both High-availability Seamless Redundancy (HSR) and Parallel Redundancy Protocol (PRP) provide zero fail-over time network redundancy by transmitting the same frame on two ports (Port-A and Port-B) and only forwarding the first unique packet received on these ports. HSR is designed to work in a ring topology, while PRP is designed to work in a double star LAN (LAN-A and LAN-B) topology.
To allow for the determining and discarding duplicate frames, additional protocol specific information is sent with the data frame. In HSR, the frames are identical, except for the path field in their 6 octet HSR header (tag), while the two PRP frames add a 6 octet Redundancy Control Trailer (RCT) that differs in the LanId.
Periodically, supervision frames allow supervision of the status of the redundant network, e.g. broken links can be detected.
Network devices which do not have the ability to communicate by HSR, can be connected to an HSR ring via a RedBox, i.e. redundancy box. In PRP, network devices can be connected to a single LAN (LAN-A or LAN-B) without network redundancy properties or also through a RedBox.