If you think back to the details of STP operation in Chapter 1, STP uses two types of numbers for most of its decisions: the BID and STP port costs. Focusing on those two types of numbers, consider this summary of what STP does behind the scenes:
Use the BID to elect the root switch, electing the switch with the numerically lowest BID.
Use the total STP cost in each path to the root, when each nonroot switch chooses its own root port (RP).
Use each switch’s root cost, which is in turn based ...