Improving the sustainability of city logistics involves the design and implementation of solutions that lead to improvements from social, environmental and economic perspectives. In this paper we focus on the ex ante sustainability assessment of city logistics solutions. For demonstration purposes, we rely on a simple fictitious city and investigate the implementation of alternative logistics pooling solutions. Using sustainability key performance indices, we contrast a centralized pooling solution relying on a single Urban Consolidation Center (UCC) with an interlinked pooling solution rather relying on a network of distributed UCCs. Key learnings are (1) the importance of rigorous ex ante sustainability assessment of proposed city logistics solutions, as the assessment investigated solutions revealed counterintuitive results, and (2) the importance of designing city logistics solution using an open interconnected systemic approach rather than a piecemeal approach, as a fundamental root cause of the results with the proposed alternative solutions was revealed by contrasting the foundations of their design to those inherent interconnected city logistics as prescribed according to the Physical Internet concept. This work is the beginning of a core project aiming to enable and to investigate the ex ante assessment of city logistics solutions for actual cities.