The intra-domain mobility management protocol (IDMP) has recently been proposed as a protocol for managing IP mobility within a cellular access network. The paper investigates the scalability performance of IDMP's quality of service (QoS) framework, which uses a modified form of the differentiated services (DiffServ) architecture, with a centralized bandwidth broker (BB) performing admission control and resource provisioning for different traffic classes. The theoretical analysis shows that requests for bandwidth reservation due to intra-domain mobility should be controlled to alleviate the processing burden at the BB. Accordingly, we propose a scalable bandwidth reservation scheme. By reserving the bandwidth in the trunk instead of on a per-host basis, QoS-related signaling load and handoff latency can be reduced significantly.