The application of federated domain generalization in person re-identification (FedDG-ReID) aims to enhance the model's generalization ability in unseen domains while protecting client data privacy. However, existing mainstream methods typically rely on global feature representations and simple averaging operations for model aggregation, leading to two limitations in domain generalization: (1) Using only global features makes it difficult to capture subtle, domain-invariant local details (such as accessories or textures); (2) Uniform parameter averaging treats all clients as equivalent, ignoring their differences in robust feature extraction capabilities, thereby diluting the contributions of high-quality clients. To address these issues, we propose a novel federated learning framework—Federated Aggregation via Robust and Discriminative Knowledge Selection and Integration (FedARKS)—comprising two mechanisms: RK (Robust Knowledge) and KS (Knowledge Selection). In our design, each client employs a dual-branch network of RK: the Global Feature Processing Branch serves as the primary component, extracting overall representations for model aggregation and server-side updates; while the Body Part Processing Branch acts as an auxiliary component, focusing on extracting domain-invariant local details to supplement and guide the local training process during global feature learning. Additionally, our KS mechanism adaptively assigns corresponding aggregation weights to clients based on their ability to extract domain-invariant knowledge, enabling the server to better integrate cross-domain invariant knowledge extracted by clients. Extensive experiments validate that FedARKS achieves state-of-the-art generalization results on the FedDG-ReID benchmark, demonstrating that learning subtle body part features can effectively assist and reinforce global representations, thereby enabling robust cross-domain person ReID capabilities.