Identifying Root Ports in Spanning Tree Protocols

Identifying Root Ports 

The second step in the STP convergence process is to identify root ports.  The root port of each switch has the lowest root path cost to get to the Root Bridge. 
Each switch can only have one root port. The Root Bridge cannot have a root port, as the purpose of a root port is to point to the Root Bridge.
Path cost is a cumulative cost to the Root Bridge, based on the bandwidth of the links. The higher the bandwidth, the lower the path cost:
                     
                                   Bandwidth                   Cost
                                    4 Mbps                         250
                                   10 Mbps                        100
                                   16 Mbps                         62
                                   45 Mbps                         39
                                   100 Mbps                       19
                                   155 Mbps                       14
                                   1 Gbps                            4
                                   1 Gbps                            2

A lower cost is preferred. Consider the following example:

Each 1Gbps link has a path cost of 4. SwitchA has a cumulative path cost of 0, because it is the Root Bridge. Thus, when SwitchA sends out BPDU’s, it advertises a root path cost of 0.