ABB UniGear Digital, Engineering Manual

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3.5.3 Topologies
SINGLE network using Rapid Spanning Tree Protocol (RSTP) / Fast Media Redundancy
Protocol (E-MRP)
The single network topology is the most common one, protection relays are connected to
managed Ethernet switch via single connection. The managed Ethernet switches form a
physical loop (ring).
RSTP ring offer redundancy mechanism against link between switches failures, but not
against protection relay link or switch failure. Rapid Spanning Tree protocol always blocks
one path to avoid duplicates. Moreover, the RSTP ring cannot guarantee a zero or near-zero
frame loss upon network failure occurrence. It is supported by AFS and RUGGEDCOM family.
Figure 81: RSTP ring redundant structure