If no bridge priority is configured in PVST (Per VLAN Spanning Tree), the criteria considered when electing the root bridge is the lowest MAC address of the switches. This happens because the bridge ID (BID) used in the election consists of the bridge priority and the MAC address. When all switches have the default priority (since none is configured), the election is determined by the switch with the lowest MAC address, as it has the lowest BID. This ensures that the switch with the lowest MAC address becomes the root bridge in the VLAN when bridge priorities are equal or not set.