Chapter 5Setup and ConfigurationRUGGEDCOM ROSUser Guide168 Configuring eRSTP2. Configure the following parameter(s) as required:Parameter DescriptionMax Network Diameter Synopsis: { MaxAgeTime, 4*MaxAgeTime }Default: 4*MaxAgeTimeThe RSTP standard puts a limit on the maximum network sizethat can be controlled by the RSTP protocol. The network sizeis described by the term 'maximum network diameter', whichis the number of switches that comprise the longest path thatRSTP BPDUs have to traverse. The standard supported maximumnetwork diameter is equal to the value of the 'MaxAgeTime' RSTPconfiguration parameter.eRSTP offers an enhancement to RSTP which allows it to covernetworks larger than ones defined by the standard.This configuration parameter selects the maximum supportednetwork size.BPDU Guard Timeout Synopsis: 1 to 86400 s or { Until reset, Don't shutdown }Default: Don't shutdownThe RSTP standard does not address network security. RSTP mustprocess every received BPDU and take an appropriate action.This opens a way for an attacker to influence RSTP topology byinjecting RSTP BPDUs into the network.BPDU Guard is a feature that protects the network from BPDUsreceived by a port where RSTP capable devices are not expectedto be attached. If a BPDU is received by a port for which 'Edge'parameter is set to 'TRUE' or RSTP is disabled, the port will beshutdown for the time period specified by this parameter.• DON'T SHUTDOWN - BPDU Guard is disabled• UNTIL RESET - port will remain shutdown until the port resetcommand is issued by the userFast Root Failover Synopsis: { On, On with standard root, Off }Default: OnIn mesh network topologies, the standard RSTP algorithm doesnot guarantee deterministic network recovery time in the case ofa root switch failure. Such a recovery time is hard to calculate andit can be different (and may be relatively long) for any given meshtopology.This configuration parameter enables Siemens's enhancement toRSTP which detects a failure of the root switch and performs someextra RSTP processing steps, significantly reducing the networkrecovery time and making it deterministic.NOTE• This feature is only available in RSTP mode. InMSTP mode, the configuration parameter isignored.• In a single ring topology, this feature is notneeded and should be disabled to avoid longernetwork recovery times due to extra RSTPprocessing.The Fast Root Failover algorithm must be supported by allswitches in the network, including the root, to guarantee optimalperformance. However, it is not uncommon to assign the root roleto a switch from a vendor different from the rest of the switchesin the network. In other words, it is possible that the root mightnot suport the Fast Root Failover algorithm. In such a scenario,a "relaxed" algorithm should be used, which tolerates the lack ofsupport in the root switch.
