To expand an enterprise network across regional branches, this study designs a network expansion solution using the highly scalable ADVPN technology, ensuring minimal disruption to the existing network architecture. Initially, network deployment at headquarters is executed on the foundational architecture of GRE over IPSec VPN. Subsequently, smart links and monitoring links are configured for the branch networks, and three-layer network routing is enabled. Moreover, the VPN address management protocol for server and client configuration is implemented on the boundary network, coupled with dynamic network address translation to facilitate Internet access for branches, thus achieving interconnection between branches and the headquarters as well as among the branches themselves. Each step of this process is validated through simulation experiments to ascertain the effectiveness and feasibility of this enterprise network expansion design.