Families of symmetric periodic orbits in 2:1 MMR of the 2D-ERTBP when m_P = m_J are overplotted on the DS maps of the plane (e_A, e_P). The white crosses represent the chosen periodic orbits for our study, the orbital elements of which remain fixed for the computation of each map. We use a positive (or negative) value for e_A when θ₁ equals 0 (or π) in the family of each configuration. The same convention applies to e_P and θ₂. Dark (pale) regions correspond to stable (chaotic) domains in phase space as attributed by the logarithmic values of the DFLI shown by the colored bar. Blue (red) curves stand for the horizontal stability (instability) of the periodic orbits, while the solid (dashed) curves show the vertical stability (instability). The magenta dot corresponds to a transition of the vertical stability. The dashed gray curve depicts the collision line between the planet and the asteroid.