Dynamic Routing Gateway (DRG) in the source region: This is mandatory. The DRG in the source region acts as a central hub for routing traffic towards the destination VCN in the other region.
Dynamic Routing Gateway (DRG) in the destination region: This is also mandatory. The DRG in the destination region receives the routing information from the source DRG and directs traffic towards the target VCN.
FastConnect or Site-to-Site VPN connection between source and destination DRGs: This establishes the physical connection between your on-premises network and the OCI regions, allowing for routing exchange and data transfer.
Internet Gateway (IGW) in the source region: An IGW provides public internet access from a VCN, but it has no role in establishing private transitive routing towards a VCN in another region. Transitive routing involves private connections using a dedicated gateway like a DRG, not public internet resources.
Therefore, while IGWs play a valuable role in enabling public internet access within a VCN, they are not required for establishing private transitive routing connections between on-premises networks and VCNs in different OCI regions. DRGs, FastConnect/VPN connections, and proper route configuration across DRGs are the essential components for achieving this type of connectivity.