Maintaining and Configuring STP
What is a backup root bridge?

A backup (or secondary) root bridge is a switch that is preferentially configured to assume the role of the root bridge in the event that the primary root bridge fails.

If no backup root bridge is configured and the root bridge fails, some other switch will be automatically chosen as the root bridge by the STP. However, it is likely that this automatic choice will not be optimal for network performance and stability.

The backup root bridge is configured to have a priority value set lower than the default, but higher than the primary root bridge. In normal operation, when the primary root bridge is functioning, the backup root bridge behaves like any other non-root bridge. When the primary root bridge fails, the backup root bridge then has the lowest priority in the network and so is selected to be the root bridge.

Configuration of an appropriate backup root bridge assures optimal network forwarding and stability in the event of the primary root bridge failure.