...
| root path selection | |
non-root switch receives periodic STP BPDUs that contain "root path cost" ( calculate by the neighbor) | |
| Local switch add the "port cost" to the root path cost | Lowest root path cost |
| the interface with the lowest root path cost become the root port | Lowest Port Cost |
| if same port cost | |
| port id is the tier breaker | Lowest port id |