🚀 Saving a $1M Integration: Why We Pivoted to AWS Transit Gateway

Source: Dev.to

The Stakes: 48 Hours to “The Swap”

We were two days away from the production cutover for a massive integration between our SaaS platform and a Fortune 500 utility provider. The goal was to migrate their legacy call center to a private cloud environment.

The integration involved a global communications partner and a high‑compliance utility environment, requiring a bridge between an AWS environment and a massive private cloud.

The Conflict: The 10.0.0.0/8 Trap

Our internal environment used the standard 10.0.0.0/16. It worked perfectly—until we tried to talk to the client.

The partner’s internal network owned the entire 10.0.0.0/8 private range. Because a /16 (our VPC) is more specific than a /8 (their backbone), BGP would have prioritized our VPC for any traffic matching that range.

Disaster scenario: Advertising our 10.0.0.0/16 would have hijacked the partner’s global internal traffic, black‑holing packets meant for offices worldwide, potentially causing a global outage just 48 hours before cutover.

The Strategy: The Secondary CIDR “Bridge”

Changing the entire VPC range overnight would have broken every internal service. Instead, we performed a surgical architectural pivot: Secondary CIDR Implementation.

• Added 172.16.0.0/16 as a secondary CIDR block to the existing VPC.
• Kept internal services running on the original 10.0.0.0/16.
• Provisioned a new subnet 172.16.1.0/24 specifically for the integration.
• When our servers communicated with the partner, they used 172.16.x.x addresses, a range that did not conflict with the partner’s global backbone.

The Pivot: From “Tunnel” to “Governance”

To make the bridge work we switched from a standard Virtual Private Gateway (VGW) to an AWS Transit Gateway (TGW).

• VGW – a passive pipe.
• Transit Gateway – an intelligent cloud router that provides granular control.

Using TGW we:

• Suppressed the 10.0.0.0/16 – ensured the internal range was never advertised over the VPN.
• Injected the 172.16.1.0/24 – advertised only the new “bridge” range to the partner.

The Technical Execution

# Example verification command
nc -zv 10.201.12.34 443   # test connectivity from 172.16.x.x to partner host

Lessons Learned for Cloud Architects

• Scale dictates architecture – A VGW suffices for simple site‑to‑site links, but when connecting to a partner with a massive legacy footprint, a Transit Gateway is essential for route filtering.
• Don’t rebuild, extend – Adding a Secondary CIDR is often faster and safer than a full VPC migration when IP conflicts arise.
• Specific beats general – In BGP, the most specific route always wins. Without controlling advertisements, a small VPC can unintentionally claim traffic meant for a global data center.

Final Thoughts

We executed “The Swap” on schedule. By pivoting to a Secondary CIDR and using a Transit Gateway for route governance, we moved the project forward without risking the partner’s global stability.

The best DevSecOps war stories are the ones where the disaster never happens because of a well‑timed architectural pivot.