This paper presents FBA-SDN, a novel Stellar Consensus Protocol (SCP)-based Federated Byzantine Agreement System (FBAS) approach to trustworthy Collaborative Intrusion Detection (CIDS) in Software-Defined Network (SDN) environments. The proposed approach employs the robustness of Byzantine Fault Tolerance (BFT) consensus mechanisms and the decentralized nature of blockchain ledgers to coordinate the Intrusion Detection System (IDS) operation securely. The federated architecture adopted in FBA-SDN facilitates collaborative analysis of low-confidence alert data, reaching system-wide consensus on potential intrusions. Additionally, the Quorum-based nature of the approach reduces the risk of a single point of failure (SPoF) while simultaneously improving upon the scalability offered by existing blockchain-based approaches. Through simulation, we demonstrate promising results concerning the efficacy of reaching rapid and reliable consensus on both binary and multi-class simulated intrusion data compared with the existing approaches.