In this paper, we adopt an energy perspective to analyze the stability of converter-dominated dc grids under a hierarchical (primary/secondary) optimal control strategy based on distributed communications, and its robustness against cyber-threats. First, we begin by showing that both the decentralized droop-controlled de microgrid, as well as the distributed secondary controller, admit a port-Hamiltonian description. Second, we exploit the fact that the closed-loop system can be interpreted as a lossy-interconection between their (incremental) models to prove stability for the unperturbed system. Third, we analyze the effect of malicious attacks on the (linear) system and robustify the control system such that resilience against cyber threats always is ensured at steady state. Additionally, we show that by adequately tuning the controllers we are able to significantly reduce the attack influence on the desired steady state. Finally, we use time-domain simulations to support our findings in a case study involving a low-voltage DC microgrid.