The arrival of mega-constellations with inter-satellite links marks a major shift in space communication, evolving toward large-scale networking systems. This spurred significant interest in creating new routing mechanisms designed to tackle the unique challenges of space-based networks, characterized by predictable but high-latency and predictably changing links. This paper proposes a Software Defined Networks (SDN)-based routing solution for mega-constellations, designed to significantly cut routing convergence time and reduce processing demands on space nodes. It relies on a centralized terrestrial controller that asynchronously computes routing information and deploys it to space nodes prior to its usage. These nodes are equipped with lightweight semantic routing capabilities that allows them to autonomously determine optimal routing based on their current network status. As any SDN solution, the design is flexible enough to accommodate various routing optimizations available in the literature, facilitating their practical implementation. The paper provides a detailed description of the proposed solution, assesses its feasibility using an example routing algorithm, and compares it with existing routing approaches, highlighting its strong potential for real-world commercial system application.