eRSTP - enhanced Rapid  Spanning Tree Protocol 4

Ethernet switches operate by forwarding traffic between their ports. The switch examines each Ethernet frame and records (learns) its MAC address and the port upon which it resides. When a frame arrives for a given MAC address, the switch "knows" on which outgoing port to send it. If a frame arrives and its destination MAC address is unknown, the switch will "flood" the frame out all of its ports.

If switches in the network are connected in a loop, a ‘broadcast storm’ will result where a single broadcast frame will circulate endlessly. This condition consumes all available bandwidth on the loop making the network unusable. The IEEE 802.1D Spanning Tree Protocol (STP) is used to prevent this situation.


STP and RSTP

The Spanning Tree Protocol (IEEE 802.1D) was designed to solve the fundamental problem of traffic loops. It achieves this by having all switches in the network supporting STP communicate to one another using a process which will identify any loops if they exist. 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, STP’s performance in terms of fault recovery time is too slow (i.e. 100’s of seconds) for most real-time control applications. In fact people in the IT world even found it too slow for office networks.

The next evolution of the protocol came in the form of IEEE 802.1w Rapid Spanning Tree Protocol (RSTP). This version of the protocol greatly improved the fault recovery performance (i.e. 10’s of seconds) but was still too slow for most real-time control applications. As with Spanning Tree Protocol (STP), it was ideally designed for mesh network topologies and not optimized for ring networks. RSTP did not support large ring sizes exceeding 40 switches.


Evolution of eRSTP

Siemens approach was to build upon the RSTP standard and enhance it in two ways:

  1. Improve the fault recovery time performance

  2. Improve performance for large ring network topologies

Siemens enhanced Rapid Spanning Tree Protocol (eRSTP) provides fault recovery times of less than 5ms per hop and can handle rings with up to 160 switches.

Another goal was maintaining compatibility with standard RSTP for interoperability with commercial switches.


eRSTP Performance Example

For a network comprised of 15 RUGGEDCOM Ethernet switches (i.e. S1 – S15) in a ring topology we can expect better than 5ms/hop fault recovery performance:

                                                   Expected fault recovery time < 75ms (i.e. 5ms x 15)

Testing performed on  RUGGEDCOM Ethernet switches with enhanced Rapid Spanning Tree Protocol technology using an industry standard SmartBits network analyzer revealed:

                                                  Actual worst case fault recovery time of < 26ms!

The 5ms/hop figure is a very conservative figure which will work for predicting fault recovery times for ring diameters up to 80 switches.