In the debate on metropolitan governance, the influence of the cantonal border has been given little attention so far. However, the need for action is particularly high in urban border regions where there are typically few contacts among the municipalities across the border. Thus, many border regions inevitably turn into peripheral regions. The aim of this article is to make a contribution to closing this research gap. It analyses the influence of a cantonal border on the municipal collaboration in the policy field of spatial planning. To this end, three urban agglomerations in the metropolitan area of Zurich are being compared: the urban region Limmattal and the urban agglomerations of Rapperswil-Jona-Rüti and Schaffhausen. These case studies point out the crucial components of the restraining effect of the cantonal border. With respect to the empirical findings, promising strategies for an improved municipal cooperation across the cantonal border are identified.