Invoice rules for allowing discount to recipients

Dear Sir,

Please guide me regarding allowing discount to our buyers in the invoice.

If we allow discount in the invoice, on which amount GST has to be calculated?

For example,

Taxable value. Rs. 100000/-
Less Discount Rs. 2000/-
Net Value Rs. 98000/-

We have to calculate the GST on
Rs. 100000/- or on Rs. 98000/-

Please guide me.......