You are designing a hybrid cloud architecture connecting your on-premises network to OCI.

Objective: Allow VCN-A to access on-premises (192.168.1.0/24) via VPN, isolate VCN-B using DRG route tables effectively and securely.

Option A: Single route table for both VCNs with NSGs on VCN-B to block traffic. This works but relies on NSGs, which are secondary to routing. Routing-level isolation is more secure and efficient.

Option B: Single route table for VCN-A with the VPN route, default table (no VPN route) for VCN-B. This isolates VCN-B effectively at the routing level, but managing one table across all attachments can complicate scaling.

Option C: Two route tables, both with VPN routes, then blocking VCN-B with security lists. This is inefficient—routes are advertised unnecessarily, relying on security lists instead of routing isolation.

Option D: Two route tables—DRG-RT-A with VPN route for VCN-A, DRG-RT-B with no VPN route for VCN-B. This ensures VCN-B has no path to on-premises at the DRG level, providing the strongest isolation.

Conclusion: Option D is the most effective and secure, leveraging routing for isolation rather than secondary security controls.

Oracle documentation states:

"DRG route tables control traffic between VCN attachments and external connections (e.g., VPN). Associate a unique route table with each attachment to enforce specific routing policies."

"To isolate a VCN, ensure its DRG route table contains no routes to the destination."Option D aligns with this approach. Reference:Dynamic Routing Gateway Overview - Oracle Help Center(docs.oracle.com/en-us/iaas/Content/Network/Tasks/managingDRGs.htm).