In this article, we study the optimal simple monetary policy rules under a Zero Lower Bound (ZLB) using a DSGE model. The modeled economy is open and highly dependent on the terms of trade (TOT). Economic dynamics is the result of a TOT shock and an external interest rate shock. Using impulse response functions, we show that the presence of the ZLB reduces the impact of positive external shocks. This means greater growth in real interest rates and lesser growth in consumption and production. The monetary authority minimizes the volatility of key macroeconomic indicators. The optimal parameters for the rule turn out to be such that the regulator de facto reduces the probability of being at the ZLB.