(Optional) Configuring the Root Bridge and Secondary Root Bridge

Context

MSTP can calculate the root bridge or you can manually configure the root bridge or secondary root bridge. Manually configuring the root bridge and secondary root bridge is recommended.

A switch can function as a root bridge or a secondary root bridge in a spanning tree. It can also function as the root bridge or secondary root bridge of another spanning tree. In a spanning tree:

Only one root bridge takes effect. If two or more root bridges are specified in a spanning tree, the device with the smallest MAC address is used.
Multiple secondary root bridges can be specified. If the root bridge fails or is powered off and no new root bridge is specified, the secondary root bridge with smallest MAC address will become the root bridge of the spanning tree.

Procedure

Perform the following operations on the device to be used as the root bridge.
1. Run system-view
  The system view is displayed.
2. Run stp [ instance instance-id ] root primary
  The device is configured as the root bridge.
  
  By default, a switch does not function as the root bridge. After the configuration is complete, the priority value of the device is 0 (this value cannot be modified).
  
  If instance is not specified, the device in MSTI 0 is a root bridge.
Perform the following operations on the device to be used as the secondary root bridge.
1. Run system-view
  The system view is displayed.
2. Run stp [ instance instance-id ] root secondary
  The device is configured as the secondary root bridge.
  
  By default, a switch does not function as the secondary root bridge. After the configuration is complete, the priority value of the device is 4096 (this value cannot be modified).
  
  If instance is not specified, the device in MSTI 0 is a secondary root bridge.