High-availability Seamless Redundancy

High-availability Seamless Redundancy (HSR) is a network protocol for Ethernet that provides seamless failover against failure of any single network component. PRP and HSR are independent of the application-protocol and can be used by most Industrial Ethernet protocols in the IEC 61784 suite. HSR does not cover the failure of end nodes, but redundant nodes can be connected via HSR.

Not to be confused with Hot Standby Router Protocol.

HSR nodes have two ports and act as a bridge, which allows arranging them into a ring or meshed structure without dedicated switches. This is in contrast to the companion standard Parallel Redundancy Protocol (PRP), with which HSR shares the operating principle. PRP and HSR are standardized by the IEC 62439-3:2016.

PRP and HSR are suited for applications that request high availability and short switchover time. For such applications, the recovery time of commonly used protocols such as the Rapid Spanning Tree Protocol (RSTP) is too long. It has been adopted for electrical substation automation in the framework of IEC 61850. It is used in synchronized drives (e.g. in printing machines) and high power inverters.

The cost of HSR is that nodes require hardware support (FPGA or ASIC) to forward or discard frames within microseconds. This cost is compensated because no Ethernet switches are required. Hardware support is anyhow needed when the node supports clock synchronization or security.

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