Configuring MSTP 135Perform the following configuration in system view.After a switch is configured as the primary root bridge or the secondary rootbridge, users cannot modify the bridge priority of the switch.You can configure the current switch as the primary or secondary root bridge ofthe STI (specified by the instance instance-id parameter). If the instance-id takes0, the current switch is specified as the primary or secondary root bridge of theCIST.The root types of a switch in different STIs are independent of one another. Theswitch can be a primary or secondary root of any STI. However, it cannot serve asboth the primary and secondary roots of one STI.If the primary root is down or powered off, the secondary root will take its place,unless you configure a new primary root. Of two or more configured secondaryroot bridges, MSTP selects the one with the smallest MAC address to take theplace of the failed primary root.When configuring the primary and secondary switches, you can also configure thenetwork diameter and hello time of the specified switching network. For detailedinformation, refer to the configuration tasks "Configure switching networkdiameter" and "Configure the Hello Time of the switch".n You can configure the current switch as the root of several STIs. However, it is notnecessary to specify two or more roots for an STI. In other words, do not specifythe root for an STI on two or more switches.You can configure more than one secondary root for a spanning tree throughspecifying the secondary STI root on two or more switches.Generally, you are recommended to designate one primary root and more thanone secondary root for a spanning tree.By default, a switch is neither the primary root nor the secondary root of thespanning tree.Configuring the MSTPRunning ModeMSTP and RSTP are compatible and they can recognize the packets of each other.However, STP cannot recognize MSTP packets. To implement the compatibility,MSTP provides two operation modes, STP-compatible mode and MSTP mode. InSTP-compatible mode, the switch sends STP packets via every port. In MSTP mode,Table 108 Specify the switch as a primary or a secondary root bridgeOperation CommandSpecify the current switch as the primary rootbridge of the specified spanning treestp [ instance instance-id ] root primary [bridge-diameter bridgenum ] [ hello-timecenti-senconds ]Specify the current switch as the secondaryroot bridge of the specified spanning treestp [ instance instance-id ] root secondary [bridge-diameter bridgenum ] [ hello-timecenti-senconds ]Specify current switch not to be the primary orsecondary root undo stp [ instance instance-id ] root
