Q: Your VPC has overlapping CIDR blocks with an on-premises network and you need to connect them via VPN. What do you do?
Overlapping CIDRs are a real problem — traffic routing becomes ambiguous.
#AWS #Networking & VPC #L3 #Cloud #Infrastructure #EC2
🎙️ Candidate Opening & Architectural Context
""AWS reliability requires differentiating between AWS control plane limits and host-level resource exhaustion. When addressing this question, I walk the interviewer through our production incident runbook: isolating the blast radius, checking diagnostic logs and metrics, and applying a safe fix.""
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🛠️ Production Runbook & Step-by-Step Resolution
1️⃣
Initial Diagnostics & Root Cause Analysis
Overlapping CIDRs are a real problem — traffic routing becomes ambiguous.
- Re-IP the VPC — If the VPC is new/small, change the CIDR by creating a new VPC with a non-overlapping range and migrating.
- NAT at the VPN gateway — Use a NAT device that translates VPC IPs to a non-overlapping range before traffic crosses the VPN. AWS doesn't natively support NAT-T for VPN in this case; you'd use an EC2-based NAT instance.
- AWS Transit Gateway with NAT — TGW supports NAT-based routing for overlapping CIDRs in some configurations.
2️⃣
Remediation & Permanent Safeguards
Options: Best practice: Plan CIDR ranges before creating VPCs. Use RFC1918 ranges with /16 subnets, ensuring no overlap between VPCs and on-prem. Document in a CMDB.
💡 The Senior SRE Gold Nugget (Key Architectural Takeaway)
"Pro-Tip: Re-IP the VPC — If the VPC is new/small, change the CIDR by creating a new VPC with a non-overlapping range and migrating.."
⚡ 60-Second Elevator Pitch Talking Points
- Re-IP the VPC — If the VPC is new/small, change the CIDR by creating a new VPC with a non-overlap...
- NAT at the VPN gateway — Use a NAT device that translates VPC IPs to a non-overlapping range befo...
- AWS Transit Gateway with NAT — TGW supports NAT-based routing for overlapping CIDRs in some confi...
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