Federated embedded systems (FES) is an approach for systems-of-systems engineering in the domain of cyber-physical systems. It is based on the idea to allow dynamic addition of plug-in software in the embedded system of a product, and through communication between the plug-ins in different products, it becomes possible to build services on the level of a federation of products. In this paper, architectural concerns for FES are elicited, and are used as rationale for a number of decisions in the architecture of products that are enabled for FES, as well as in the application architecture of a federation. A concrete implementation of a FES from the automotive domain is also described, as a validation of the architectural concepts presented.