  We present the dynamical structure of the phase space of the planar planetary
2/1 mean-motion resonance (MMR). Inside the resonant domain, there exist two
families of periodic orbits, one associated to the librational motion of the
critical angle ($\sigma$-family) and the other related to the circulatory
motion of the angle between the pericentres ($\Delta\varpi$-family). The
well-known apsidal corotation resonances (ACR) appear at the intersections of
these families. A complex web of secondary resonances exists also for low
eccentricities, whose strengths and positions are dependent on the individual
masses and spatial scale of the system.
  Depending on initial conditions, a resonant system is found in one of the two
topologically different states, referred to as \textit{internal} and
\textit{external} resonances. The internal resonance is characterized by
symmetric 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 and
longitudes of pericentre, respectively. In contrast, the external resonance is
characterized by asymmetric ACR and the resonant angle is
$2\,\lambda_2-\lambda_1-\varpi_2$. We show that systems with more massive outer
planets always envolve inside internal resonances. The limit case is the
well-known asteroidal resonances with Jupiter. At variance, systems with more
massive inner planets may evolve in either internal or external resonances; the
internal resonances are typical for low-to-moderate eccentricity
configurations, whereas the external ones for high eccentricity configurations
of the systems. In the limit case, analogous to Kuiper belt objects in
resonances with Neptune, the systems are always in the external resonances
characterized by asymmetric equilibria.
