Unidirectional links brought by many receive-only stations are not suitable for current routing protocols. To solve the unidirectional routing problem, a network model is built for the topology of direct broadcast systems. By simplifying the network model, a link state routing algorithm based on circle discovery and a new protocol SERP (sever-based routing protocol) are proposed. By means of proven convergency of the routing algorithm and simulating results from network simulator tool for the protocol, the correctness of SERP is verified,which has fewer ovwer overheads and supports dynamic routing for integrating broadband satellite networks with high spped Interet.