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We study the worldsheet reflection matrix of a string attached to a D-brane in $AdS_5 \times S^5$. The D-brane corresponds to a maximal giant graviton and it wraps an $S^3$ inside $S^5$. In the gauge theory, the open string is described by a spin chain with boundaries. We study an open string with a large SO(6) charge, which allows us to focus on one boundary at a time and to define an asymptotic boundary reflection matrix. We consider two cases corresponding to two possible relative orientations for the charges of the giant graviton and the open string. Using the symmetries of the problem we compute the boundary reflection matrix up to a phase. These matrices obey the boundary Yang Baxter equation. A crossing equation is derived for the overall phase. We perform weak coupling computations up to two loops and obtain results that are consistent with integrability. Finally, we determine the phase factor at strong coupling using classical solutions.