  We show that only a few percentage of Sn doping at the Ba site on
BaFe$_2$As$_2$, can cause electronic topological transition, namely, the
Lifshitz transition. A hole like d$_{xy}$ band of Fe undergoes electron like
transition due to 4\% Sn doping. Lifshitz transition is found in BaFe$_2$As$_2$
system around all the high symmetry points. Our detailed first principles
simulation predicts absence of any Lifshitz transition in other 122 family
compounds like SrFe$_2$As$_2$, CaFe$_2$As$_2$. This work bears practical
significance due to the facts that a few percentage of Sn impurity is in-built
in tin-flux grown single crystals method of synthesizing 122 materials and
inter-relationship among the Lifshitz transition, magnetism and
superconductivity.
