eRSTP™ (enhanced Rapid Spanning Tree Protocol) 

Ethernet switches operate by forwarding traffic between their ports. The switch examines each Ethernet frame and records its MAC address and the port upon which it resides. When a frame arrives for a given MAC address, the switch knows which outgoing port to send it to. If the destination MAC address of a frame is unknown, the switch will “flood" the frame out of all of its ports. This can result in a “broadcast storm” if the switches are connected in a loop. 

The IEEE 802.1D Spanning Tree Protocol (STP) was designed to solve the problem of traffic loops by having all switches in a network communicate with one another using a process which would identify any existing loops. Once a loop is identified, it is logically broken by having one of the switches block communications from the port which creates the loop. However, this protocol’s fault recovery time was too slow for most real-time control applications. 

Rapid Spanning Tree Protocol (RSTP) greatly improved the fault recovery performance from 100s of seconds to 10s of seconds but was still too slow for most real-time control applications. As with the Spanning Tree Protocol (STP), it was ideally designed for mesh network topologies and not ring networks. This protocol also did not support large ring sizes exceeding 40 switches.

Siemens approach was to build upon the RSTP standard and enhance it in two ways: by improving the fault recovery time, and by improving performance for large ring network topologies.

Called enhanced Rapid Spanning Tree Protocol (eRSTP), this protocol provides fault recovery times of less than 5 ms per hop and can handle rings with up to 160 switches. This protocol is also compatible with standard RSTP for commercial interoperability.