IEC 62439-3:2021 specifies two related redundancy protocols that, in the event of failure of any network element, provide seamless switchover with zero recovery time:
• PRP (Parallel Redundancy Protocol), which allows attaching nodes to two separate networks while allowing attachment of nodes to one network only; and
• HSR (High-availability Seamless Redundancy), which allows threading two-port nodes in a ring or multi-port nodes in a meshed network.