Procedure (Optional) Configuring a Secondary Root Device When you configure a device with the extended system ID support as the secondary root, the device priority is modified from the default value (32768) to 28672. The device is then likely to become the root device for the specified instance if the primary root device fails. This is assuming that the other network devices use the default device priority of 32768 and therefore are unlikely to become the root device. You can execute this command on more than one device to configure multiple backup root devices.
Use the same network diameter and hello-time values that you used when you configured the primary root device with the spanning-tree mst instance-id root primary global configuration command. To configure a secondary root device, perform this procedure: Before you begin • An MST must be specified and enabled on the device. • You must also know the specified MST instance ID. Procedure Configuring Multiple Spanning-Tree Protocol (Optional) Configuring a Secondary Root Device Command or Action Purpose Step 1 enable Example: Device> enable Enables privileged EXEC mode. Enter your password if prompted. Step 2 configure terminal Example: Device# configure terminal Enters global configuration mode.
Step 3 spanning-tree mst instance-id root primary Example: Device(config)# spanning-tree mst 0 root primary Configures a device as the root device. For instance-id, you can specify a single instance, a range of instances separated by a hyphen, or a series of instances separated by a comma. The range is 0 to 4094. Step 4 end Example: Device(config)# end Returns to privileged EXEC mode. Command or Action Purpose Step 1 enable Example: Device> enable Enables privileged EXEC mode. Enter your password if prompted. Configuring Multiple Spanning-Tree Protocol 18