  The Markov gap \cite{Hayden:2021gno}, namely the difference between reflected
entropy and mutual information, is explicitly computed in the defect extremal
surface model, JT gravity, and the generic 2d extremal black holes, in vacuum
states.
  The phases that contain various island contributions are considered, and
their existence is carefully checked.
  Moreover, we show explicitly how the Markov gap originates from the OPE
coefficient of the boundary CFT. And, as a generalization of
\cite{Hayden:2021gno}, the lower bound of the Markov gap is given by
$\frac{c}{3}\log 2$ times the number of EWCS boundaries on minimal surfaces.
  We propose a boundary way of counting the lower bound for the Markov gap,
which states that the lower bound is given by $\frac{c}{3}\log 2$ times the
number of gaps between two boundary regions in vacuum states.
  We discuss the limitation and possible generalization of the boundary
counting, and its relation to tripartite entanglement.
