1-19z There is one and only one root bridge in effect in a spanning tree instance. If two or more deviceshave been designated to be root bridges of the same spanning tree instance, MSTP will select thedevice with the lowest MAC address as the root bridge.z You can specify multiple secondary root bridges for the same instance. Namely, you can specifysecondary root bridges for the same instance on two or more than two devices.z When the root bridge of an instance fails or is shut down, the secondary root bridge (if you havespecified one) can take over the role of the primary root bridge. However, if you specify a newprimary root bridge for the instance at this time, the secondary root bridge will not become the rootbridge. If you have specified multiple secondary root bridges for an instance, when the root bridgefails, MSTP will select the secondary root bridge with the lowest MAC address as the new rootbridge.z Alternatively, you can also specify the current device as the root bridge by setting the priority of thedevice to 0. For the device priority configuration, refer to Configuring the Priority of the CurrentDevice.Configuration example# Specify the current device as the root bridge of MSTI 1 and a secondary root bridge of MSTI 2. system-view[Sysname] stp instance 1 root primary[Sysname] stp instance 2 root secondaryConfiguring the Work Mode of an MSTP DeviceMSTP and RSTP can recognize each other’s protocol packets, so they are mutually compatible.However, STP is unable to recognize MSTP packets. For hybrid networking with legacy STP devicesand for full interoperability with RSTP-enabled devices, MSTP supports three work modes:STP-compatible mode, RSTP mode, and MSTP mode.z In STP-compatible mode, all ports of the device send out STP BPDUs,z In RSTP mode, all ports of the device send out RSTP BPDUs. If the device detects that it isconnected with a legacy STP device, the port connecting with the legacy STP device willautomatically migrate to STP-compatible mode.z In MSTP mode, all ports of the device send out MSTP BPDUs. If the device detects that it isconnected with a legacy STP device, the port connecting with the legacy STP device willautomatically migrate to STP-compatible mode.Configuration procedureFollow these steps to configure the MSTP work mode:To do... Use the command... RemarksEnter system view system-view —Configure the work mode ofMSTP stp mode { stp | rstp | mstp } OptionalMSTP mode by defaultConfiguration example# Configure MSTP to work in STP-compatible mode.
