43NOTE:• Two or more spanning tree devices belong to the same MST region only if they are configured to havethe same format selector (0 by default, not configurable), MST region name, MST region revision level,and the same VLAN-to-instance mapping entries in the MST region, and they are interconnected via aphysical link.• The configuration of MST region–related parameters, especially the VLAN-to-instance mapping table,will result in a new spanning tree calculation. To reduce the possibility of topology instability, the MSTregion configuration takes effect only after you activate it by using the active region-configurationcommand, or enable a spanning tree protocol by using the stp enable command in the case that thespanning tree protocol is disabled.Configuring the root bridge or a secondary root bridgeThe root bridge of a spanning tree is determined through spanning tree calculation. Alternatively, youcan specify the router as the root bridge or a secondary root bridge.A router has independent roles in different spanning trees. It can act as the root bridge in one spanningtree and as a secondary root bridge in another. However, a router cannot be the root bridge and asecondary root bridge in the same spanning tree.A spanning tree can have one root bridge only. If two or more routers are selected as the root bridge ina spanning tree at the same time, the router with the lowest MAC address wins out.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 new primaryroot bridge for the instance then, the secondary root bridge will not become the root bridge. If you havespecified multiple secondary root bridges for an instance, when the root bridge fails, the secondary rootbridge with the lowest MAC address is selected as the new root bridge.Configuring the current router as the root bridge of a specific spanning treeTo configure the current router as the root bridge of a specific spanning tree:Step Command Remarks1. Enter system view. system-view N/A2. Configure the current router asthe root bridge.• In STP/RSTP mode:stp root primary• In MSTP mode:stp [ instance instance-id ] rootprimaryUse one of the commands.By default, a router does notfunction as the root bridge.Configuring the current router as a secondary root bridge of a specific spanning treeTo configure the current router as a secondary root bridge of a specific spanning tree:Step Command Remarks1. Enter system view. system-view N/A
