When the root bridge of spanning tree instance is lost, its information may be retained in the network until the aging mechanism causes it to be removed. This leads to a delay in convergence on for the new root. This backup root feature is used to get faster convergence when the root bridge connectivity is lost.
Backup root feature enabled bridge port should be connected to Root with point to point link. When backup root bridge lost contact with the root bridge, automatically backup root bridge lowers its bridge priority (an increment below the priority of the lost root). This will cause the backup root bridge to become the new root with information superior to any information held by any bridge in the network. While save and reboot automatically changed priority will not be saved and restored. At the time of reboots it will have its priority restored to the original configured value by administrator.
If the priority of the root bridge is Zero and backup root has lost the connectivity of the root bridge then automatic assignment of the priority value for the backup root will be the initial configured value.
This backup root feature is activated only when there is loss of connectivity with the root bridge. Raising the priority on the root will not cause the Backup root feature to be activated.
Consider the following topology: 10G link connected from the edge switches to the aggregation Root while the edge switches to the Backup Root switch with 1G links to reduce the cost and acts as a backup link. In this case, when the link between aggregation Root and Backup Root fails, the traffic will now use the 1G links providing reduced performance of the network. The aggregation old root is still in the network which can be used to forward the traffic. This feature has moved the root of the stp domain.