Sepam Ethernet Guide - Schneider - error

Sepam Ethernet GuideNetwork operationThe Spanning Tree ProtocolGuidelines for using RSTP in ringarchitecturesRoot bridge locationRSTP performance can be dramatically affected by the time required to elect the rootbridge. To obtain the best results, the root bridge must be located on the main ringwith an alternate root bridge candidate on this same ring in case the root bridge fails.The switch chosen to be the bridge must have its priority set to 4196. The alternateroot must be located at the opposite side of the ring and have its priority set to 8192.Other switches on the main ring must keep their default priority of 32768.DE80782Worst rootlocationMaximum number of switchesIn a 2 level ring architecture, such as the one shown on the left, the RSTP diametercan be computed as:DRSTP = Nmain + Nsub_max , DRSTP y 40Where:b DRSTP is the RSTP diameterb Nmain is the number of switches on the main ringb Nsub_max is the number of switches on the longer sub-ringThis formula applies only if the root switch is located on the main ring.RSTP diameter determination.By knowing the number of switches on the main ring, the maximum number ofswitches on any sub-ring can be computed as:Nsub_max y 40 - NmainIt is recommended to keep the number of switches below the maximum value. RSTPperformance is better if rings are kept short. For example, it is better to use 3 subringsof 10 devices than 2 sub-rings of 15 devices.Max Age Time parameterThis parameter must be greater than the network diameter. Its maximum value is 40,which is also the default value in the ACE850. The default value in switches is usually20.To improve performance, this parameter must be as small as possible by setting it tothe value of the RSTP diameter:DRSTP y MaxAgeTime y 40Special case for RuggedCom switchesThese switches, when configured for e-RSTP, use a proprietary protocolenhancement that enables a greater number of switches to be used. The formula thatapplies in this case is:Nsub_max y (MaxAgeTime - 1) - [(Nmain - 1) / 4]DE80783Example of performanceThe graph on the left shows the measured recovery time (Y axis, in ms) related tothe number of ACE850 devices (X axis), on a single ring in case of link failure.The actual performance level depends on the topology, the nature of the event, andthe types of the devices on the network.RSTP performance example (link failure, single ring).30 SEPED310018EN - 10/2010