We consider an ideal chain with fixed ends which passes through a slip-link situated midway on the axis connecting the ends. When the chain is long enough, the system exhibits bistability : in equilibrium the number of segments on the two sides of the slip-link differ.(1) We show that introducing additional geometrical constraints (i.e., having the polymer inside two adjacent boxes connected by a slip-link) changes the situation : The shape of the boxes determines whether the asymmetry persists. For boxes elongated along the axis, the situation is symmetric, whereas for flat boxes, bistability may occur.