## 2 Existing Redundancy Protocols  
High seamless communication with fault tolerance is one of the key requirements for Ethernet-based, mission-critical, and real-time systems, such as substation automation systems, automation networks, and other industrial Ethernet networks. A fault-tolerant Ethernet eliminates single points of failure and therefore improves overall system availability. The Ethernet standardized by the Institute of Electricaland Electronics Engineers (IEEE) in IEEE 802.3 [21] is incapable of supporting a fault-tolerant network at all. A fault-tolerant Ethernet system is a common solution for addressing fault tolerance in Ethernet-based systems. Various fault-tolerant systems and redundancy protocols have been developed and standardized to provide fault tolerance and redundancy for Ethernet networks, such as fault-tolerant Ethernet (FTE) [22], scalable autonomous fault-tolerant Ethernet (SAFE) scheme [23], robust software-based fault-tolerant (RSAFE) scheme [24], spanning tree protocol (STP) [25], rapid spanning tree protocol (RSTP) [26], shortest path bridging (SPB) [27], transparent interconnection of lots of links (TRILL) [28; 29; 30; 31; 32], multiple MAC registration protocol (MMRP) [33], media redundancy protocol (MRP) [34], cross-network redundancy protocol (CRP) [35], beacon redundancy protocol (BRP) [36], parallel redundancy protocol (PRP) [1], high-availability seamless redundancy (HSR) [1] and others. FTE [22] was created by Honeywell to provide rapid network redundancy. Each node is connected twice to a single local area network (LAN) through dual network interfaces. The driver and FTE-enabled components allow network communication to occur over an alternate route when the primary route fails. SAFE [23] is a software-based approach that provides network scalability and autonomous fault detection and recovery. SAFE adopts a heartbeat mechanism, which periodically sends heartbeat messages for fault detection. SAFE divides large-scale networks into several subnets by limiting the number of nodes in each subnet. This network can be easily configured as a star network to meet fault recovery time requirements. RSAFE [24] is an enhanced version of SAFE. It is also based on multiple subnets architecture and therefore incorporates most of the advantages of the SAFE scheme; however, the RSAFE scheme adopts novel algorithms to improve the failure switchover delay and eliminates the master nodes in each subnet to reduce the latency in inter-subnet communications. STP [25] was standardized by the IEEE to provide redundancy in Ethernet networks. STP ensures that only one (active) path is used between any two nodes in an Ethernet LAN since logical loops are not allowed in Ethernet networks. STP can thus introduce network redundancy by having alternative paths that can be used by reconfiguring the network in case of a failure of a network link or switch. The main drawback of STP is the fairly slow network reconfiguration process, often taking about 30-50 s. This is far from satisfactory for most industrial applications. RSTP [26] has been developed as an enhancement to the original STP to provide faster network reconfiguration; however, RSTP still suffers switchover delay when failure occurs. SPB [27] and TRILL [28; 29; 30; 31; 32] were developed as alternatives to the spanning tree protocols. Both use the Intermedia System to Intermedia System (IS-IS) routing protocol instead of the spanning tree protocols and forward traffic via the shortest path between two nodes. SPB has been standardized by the IEEE as the next step in the evolution of the various spanning tree and registration protocols. SPB uses shortest path trees (SPTs) as alternatives to the spanning trees used by STP and RSTP. Shortest path trees guarantee that traffic is always forwarded via the shortest path between two nodes. SPB uses the IS-IS routing protocol to exchange information between nodes to calculate SPTs. SPB allows all paths to be active with multiple equal cost paths and provides much larger layer 2 topologies.