Optimizing STP 

Because there is a separate instance of the spanning tree for every active VLAN, a separate root election is conducted for each instance. If the default switch priorities are used in root selection, the similar root is elected for every spanning tree, as we have seen. This could lead to an inferior design. Some reasons to control the selection of the root switch include:  

• The root switch is responsible for generating BPDUs in STP 802.1D and is the focal point for spanning tree control traffic. The root switch must be capable of handling this additional processing load.
• The placement of the root defines the active switched paths in the network. Random placement is likely to lead to suboptimal paths. Ideally the root is in the distribution layer.