STP calculates the path cost based on the media speed (bandwidth) of the links between switches and the port cost of each port forwarding frame. Spanning tree selects the root port based on the path cost. The port with the lowest path cost to the root bridge becomes the root port. The root port is always in the forwarding state.
Calculating the Port Cost Using the Short Method
The IEEE 802.1D specification assigns 16-bit (short) default port cost values to each port that is based on bandwidth. You can also manually assign port costs between 1-65535. The 16-bit values are only used for ports that have not been specifically configured for port cost. Table 7-1 shows the default port cost values that are assigned by the switch for each type of port when you use the short method to calculate the port cost.
Table 7-1 Default Port Cost Values Using the Short Method