Q: Two EC2 instances in the same VPC can't communicate. What do you check?
1. Same VPC? — Confirm both are in the same VPC. Different VPCs require VPC Peering.
#AWS #Networking & VPC #L2 #Cloud #Infrastructure #EC2
🎙️ Candidate Opening & Architectural Context
""When an interviewer asks how I troubleshoot this in AWS, I frame it through my hands-on production experience. 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
- Same VPC? — Confirm both are in the same VPC. Different VPCs require VPC Peering.
- Security Groups — instance A's SG must allow inbound from instance B's IP or SG. And B's SG must allow outbound (usually default allows all outbound).
- NACL — both subnet NACLs must allow inbound and outbound traffic between them.
2️⃣
Remediation & Permanent Safeguards
Execute the resolution runbook and verify workload health:
- Route tables — both subnets must have routes to each other. In the same VPC, local routes (
10.0.0.0/16 → local) handle this automatically. - Are they in different VPCs with peering? — check the peering connection and route tables.
💡 The Senior SRE Gold Nugget (Key Architectural Takeaway)
"Pro-Tip: Same VPC? — Confirm both are in the same VPC. Different VPCs require VPC Peering.."
⚡ 60-Second Elevator Pitch Talking Points
- Same VPC? — Confirm both are in the same VPC. Different VPCs require VPC Peering.
- Security Groups — instance A's SG must allow inbound from instance B's IP or SG. And B's SG must ...
- NACL — both subnet NACLs must allow inbound and outbound traffic between them.
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