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We present the dynamical structure of the phase space of the planar planetary2/1 mean-motion resonance (MMR). Inside the resonant domain, there exist twofamilies of periodic orbits, one associated to the librational motion of thecritical angle ($\sigma$-family) and the other related to the circulatorymotion of the angle between the pericentres ($\Delta\varpi$-family). Thewell-known apsidal corotation resonances (ACR) appear at the intersections ofthese families. A complex web of secondary resonances exists also for loweccentricities, whose strengths and positions are dependent on the individualmasses and spatial scale of the system.  Depending on initial conditions, a resonant system is found in one of the twotopologically different states, referred to as \textit{internal} and\textit{external} resonances. The internal resonance is characterized bysymmetric ACR and its resonant angle is $2\,\lambda_2-\lambda_1-\varpi_1$,where $\lambda_i$ and $\varpi_i$ stand for the planetary mean longitudes andlongitudes of pericentre, respectively. In contrast, the external resonance ischaracterized by asymmetric ACR and the resonant angle is$2\,\lambda_2-\lambda_1-\varpi_2$. We show that systems with more massive outerplanets always envolve inside internal resonances. The limit case is thewell-known asteroidal resonances with Jupiter. At variance, systems with moremassive inner planets may evolve in either internal or external resonances; theinternal resonances are typical for low-to-moderate eccentricityconfigurations, whereas the external ones for high eccentricity configurationsof the systems. In the limit case, analogous to Kuiper belt objects inresonances with Neptune, the systems are always in the external resonancescharacterized by asymmetric equilibria.

The periodic and chaotic regimes of motion in the exoplanet 2/1 mean-motion resonance