Influenced by end-host overlay systems, the overlay multicast routing problem is studied under a singlehomed model in most literatures. This model is no longer applicable to those fixed MSNs(multicast service node), which may have multiple access links to the physical network. In order to solve the problem correctly, we propose a multihomed proxy model in this paper to describe the overlay network more accurately. We also develop a general solution to the routing problem under our model, which includes a path selection process and a multicast routing process. In the end we design an OMMP (overlay multicast over multihomed proxy) architecture, which integrates all MSNs into one framework to realize the overlay multicast service under our model.