An eikonal algebra C(black-letters)(Ω) is a C*-algebra related to a metric graph Ω. It is determined by trajectories and reachable sets of a dynamical system associated with the graph. The system describes the waves, which are initiated by boundary sources (controls) and propagate into the graph with finite velocity. Motivation and interest to eikonal algebras comes from the inverse problem of reconstruction of the graph via its dynamical and/or spectral boundary data. Algebra C(black-letters)(Ω) is determined by these data. In the mean time, its structure and algebraic invariants (irreducible representations) are connected with topology of Ω. We demonstrate such connections and study C(black-letters)(Ω) by the example of Ω of a simple structure. Hopefully, in future, these connections will provide an approach to reconstruction.