In general, bangap will generate a voltage of 1.2V.
You can apply the 1.2 to a resistor to generte a current and then use current mirror to enlarger the current by 2.1/1.2. Let the current pass the same resistor you will get 2.1v voltage.
Or in the core bandgap voltage generator, you can simply mirror the PTAT current that is generated, and pass it through the required resistor. Of course, do remember that the voltage generated will be temperature dependent.