  In this work we study the optimality of increasing stability of the inverse
boundary value problem (IBVP) for Schr\"{o}dinger equation. The rigorous
justification of increasing stability for the IBVP for Schr\"{o}dinger equation
were established by Isakov \cite{Isa11} and by Isakov, Nagayasu, Uhlmann, Wang
of the paper \cite{INUW14}. In \cite{Isa11}, \cite{INUW14}, the authors showed
that the stability of this IBVP increases as the frequency increases in the
sense that the stability estimate changes from a logarithmic type to a
H\"{o}lder type. In this work, we prove that the instability changes from an
exponential type to a H\"older type when the frequency increases. This result
verifies that results in \cite{Isa11}, \cite{INUW14} are optimal.
