Multimedia conferencing is a communication service to approach multi-party, real-time, interactive and multimedia applications. Due to mature Internet technologies and ubiquitous network access services, more and more Internet applications about real-time multimedia conferencing are used. In the current conferencing systems, the media processing in centralized mixer servers is the system bottleneck. The communication quality will be decreased because of the central system loading. This paper proposes a cluster-based framework to provide a SIP-based multimedia conferencing service. In this framework, the cluster members are self-organizing and self-maintaining to share the resources of mixers within the cluster. Additionally, the extended SIP messages and SIP-based clustering mechanism are designed in this paper. Thus, if the resources of the mixers can be allocated in an optimal planning, the mixer loading can be distributed well to each mixer in the cluster. The system requirements of super-performance mixer servers can be avoided. In the framework, not only the system loading of conference servers can be distributed well, but also the infrastructure cost can be reduced.