GE MiCOM P40 Agile Technical Manual

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10 LARGE RSTP NETWORKS COMBINING STAR AND RINGThe following diagram shows a star of four rings. Each ring is connected to the root bridge. The root bridge is ahigh-end RSTP-enabled bridge with the maximum number of ports as required. The devices A1, A2…Anmax, B1,B2…Bnmax, C1, C2…Cnmax, D1, D2…Dnmax, represent the RSTP redundant Ethernet boards.The maximum number of boards that can be connected in single ring in an RSTP-enabled network depends on theMax Age parameter. The hop count from the root bridge can not be greater than the Max Age parameter.The maximum number of RSTP bridges in a ring is given byNmax = (Max Age – 1)Where:Nmax = maximum number of devices in a ringMax Age = Max Age value configured in the root bridge.In the following diagram, if the default value of Max Age is 20 seconds, the maximum number of devices that canbe connected in ring A is 19.If Max Age is configured as 40 seconds, the maximum number of IEDs that can be connected in the network is(40-1) = 39. According to the IEEE 802.1D 2004 standard, the maximum value for the Max Age parameter is limitedto 40. To use the maximum number of IEDs in the ring, the following configuration should be used.Max Age = 40 secondsForward Delay = 30 secondsHello Time = 2 secondsBridge Priority = As required by the end user.The IEEE 802.1D 2004 standard defines the relation between Max Age and Forward Delay as:2 * (Forward Delay – 1.0 seconds) >= Max AgeTo have the maximum number of nodes in the RSTP network, the number of rings can be increased, depending onthe number of ports available in the root bridge.Chapter 6 - Commissioning Instructions86 Px4x-REB-TM-EN-6