Senior DevOps / SRE [L2] Networking Production Scenario [L2]

Q: Your company acquired another startup. You need to peer their AWS VPC with yours. You try to set it up, but AWS rejects the peering connection due to "CIDR Overlap". How do you solve this so the networks can communicate?

Q: Your company acquired another startup. You need to peer their AWS VPC with yours. You try to set it up, but AWS rejects the peering connection due to \"CIDR Overlap\". How do you solve this so the networks can communicate?

\"Isolating network failures requires proving whether packets are dropped by route tables, security groups, or stateless NACLs. The interviewer is testing: IP addressing conflicts, VPNs, PrivateLink.. I structure my answer around systematic triage first, root cause analysis second, and permanent remediation third.\" Key Strategy: Pro-Tip: AWS PrivateLink: If you only need to expose specific services (e.g., API or DB), you can put an NLB in front of the startup's serv.

VPC Peering strictly prohibits routing between overlapping CIDR blocks (e.g., both VPCs use 10.0.0.0/16) because the routing tables would...

#Networking #Networking #L2 #VPC #DNS #Security

🎙️ Candidate Opening & Architectural Context

""Isolating network failures requires proving whether packets are dropped by route tables, security groups, or stateless NACLs. The interviewer is testing: IP addressing conflicts, VPNs, PrivateLink.. I structure my answer around systematic triage first, root cause analysis second, and permanent remediation third.""

🛠️ Production Runbook & Step-by-Step Resolution

1️⃣

Initial Diagnostics & Root Cause Analysis

VPC Peering strictly prohibits routing between overlapping CIDR blocks (e.g., both VPCs use 10.0.0.0/16) because the routing tables would have no way to distinguish local vs remote traffic.

AWS PrivateLink: If you only need to expose specific services (e.g., API or DB), you can put an NLB in front of the startup's service and expose it via PrivateLink to an Endpoint in your VPC. This maps their service to an IP in *your* subnet, completely bypassing the CIDR conflict.
Transit Gateway with NAT: Use AWS Transit Gateway with an intermediary VPC running a NAT or proxy instance to translate the overlapping IPs.
Re-IP: The most painful but permanent solution is migrating one of the VPCs to a new, non-overlapping CIDR block.

2️⃣

Remediation & Permanent Safeguards

To solve this:

💡 The Senior SRE Gold Nugget (Key Architectural Takeaway)

"Pro-Tip: AWS PrivateLink: If you only need to expose specific services (e.g., API or DB), you can put an NLB in front of the startup's serv."

⚡ 60-Second Elevator Pitch Talking Points

AWS PrivateLink: If you only need to expose specific services (e.g., API or DB), you can put an N...
Transit Gateway with NAT: Use AWS Transit Gateway with an intermediary VPC running a NAT or proxy...
Re-IP: The most painful but permanent solution is migrating one of the VPCs to a new, non-overlap...

Q: Your company acquired another startup. You need to peer their AWS VPC with yours. You try to set it up, but AWS rejects the peering connection due to "CIDR Overlap". How do you solve this so the networks can communicate?

🛠️ Production Runbook & Step-by-Step Resolution

Initial Diagnostics & Root Cause Analysis

Remediation & Permanent Safeguards

📚 Related Production Scenarios in Networking