  We consider fertile three-state Hard-Core (HC) models with the activity
parameter $\lambda>0$ on a Cayley tree. It is known that there exist four types
of such models: wrench, wand, hinge, and pipe. These models arise as simple
examples of loss networks with nearest-neighbor exclusion. In the case wand on
a Cayley tree of order $k\geq2$, exact critical values $\lambda>0$ are found
for which two-periodic Gibbs measures are not unique. Moreover, we study the
extremality of the existing two-periodic Gibbs measures on a Cayley tree of
order two.
