Configuring the root bridge or a secondary root bridge

You can have the spanning tree protocol determine the root bridge of a spanning tree through calculation. You can also specify the current device as the root bridge or as a secondary root bridge.

A device has independent roles in different spanning trees. It can act as the root bridge in one spanning tree and as a secondary root bridge in another. However, one device cannot be the root bridge and a secondary root bridge in the same spanning tree.

A spanning tree can have only one root bridge. If multiple devices can be selected as the root bridge in a spanning tree, the device with the lowest MAC address is chosen.

When the root bridge of an instance fails or is shut down and no new root bridge is specified, the following events occur:

• If you specify only one secondary root bridge for the instance, it becomes the root bridge.

• If you specify multiple secondary root bridges for the instance, the secondary root bridge with the lowest MAC address is given priority.

• If you do not specify a secondary root bridge, a new root bridge is calculated.

You can specify one root bridge for each spanning tree, regardless of the device priority settings. Once you specify a device as the root bridge or a secondary root bridge, you cannot change its priority.

You can configure the current device as the root bridge by setting the device priority to 0. For the device priority configuration, see "Configuring the device priority."