An implementation method of a core SONET/SDH switch with high capacity is introduced in the paper. High-speed serial I/O?switching architectures and design considerations for switching unit are involved. It supports strictly non-blocking for unicast traffic and re-arrangeably non-blocking for dual-cast. Dualcast traffic allows for efficient scheduling of working and protection paths in UPSR（Unidirectional Path Switched Ring ）/BLSR （ Bidirectional Line Switched Ring） applications.