RUGGEDCOM ROX IIUser GuideChapter 5Setup and ConfigurationRSTP in Ring Backbone Configurations 699To design a structured wiring configuration, do the following:1. Select the design parameters for the network.What are the requirements for robustness and network failover/recovery times? Are there any specialrequirements for diverse routing to a central host computer? Are there any special port redundancyrequirements?2. Identify required legacy support.Are STP bridges used in the network? These bridges do not support rapid transitioning to forwarding. If thesebridges are present, can they be re-deployed closer to the network edge?3. Identify edge ports and ports with half-duplex/shared media restrictions.Ports that connect to host computers, IEDs and controllers may be set to edge ports in order to guaranteerapid transitioning to forwarding as well as to reduce the number of topology change notifications in thenetwork. Ports with half-duplex/shared media restrictions require special attention in order to guarantee thatthey do not cause extended fail-over/recovery times.4. Choose the root bridge and backup root bridge carefully.The root bridge should be selected to be at the concentration point of network traffic. Locate the backup rootbridge adjacent to the root bridge. One strategy that may be used is to tune the bridge priority to establishthe root bridge and then tune each bridge’s priority to correspond to its distance from the root bridge.5. Identify desired steady state topology.Identify the desired steady state topology taking into account link speeds, offered traffic and QOS. Examineof the effects of breaking selected links, taking into account network loading and the quality of alternate links.6. Decide upon a port cost calculation strategy.Select whether fixed or auto-negotiated costs should be used? It is recommended to use the auto-negotiatedcost style, unless it is necessary for the network design to change the auto-negotiated cost style. Selectwhether the STP or RSTP cost style should be used. Make sure to configure the same cost style on alldevices on the network.7. Enable RSTP Fast Root Failover option.This is a proprietary feature of Siemens. In a mesh network with only RUGGEDCOM devices in the coreof the network, it is recommended to enable the RSTP Fast Root Failover option to minimize the networkdowntime in the event of a Root bridge failure.8. Calculate and configure priorities and costs.9. Implement the network and test under load.Section 5.34.2.2RSTP in Ring Backbone ConfigurationsRSTP may be used in ring backbone configurations where rapid recovery from link failure is required. In normaloperation, RSTP will block traffic on one of the links, for example, as indicated by the double bars through link Hin Figure 818. In the event of a failure on link D, bridge 444 will unblock link H. Bridge 333 will communicate withthe network through link F.
