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.