Defining the Spanning Tree Protocol – STP
Identifying the root selection process

BPDUs are exchanged between switches and the analysis of the Bridge ID and Root ID information from those BPDUs determines which bridge is selected as the root bridge.

In the example shown, both switches have the same priority for the same VLAN. The switch with the lowest MAC address will, therefore, be elected root bridge. In the example, switch X is the root bridge with a bridge ID of 0x8401:0c0011111111.  –