Q: Two physical data centers are connected via two distinct ISPs. Traffic goes out via ISP 1, but the return packets from the internet come back via ISP 2. The corporate firewall immediately drops the return packets. Why?
This is called Asymmetric Routing.
#Networking #Networking #L2 #VPC #DNS #Security
🎙️ Candidate Opening & Architectural Context
""When an interviewer asks how I diagnose network connectivity, I follow an outside-in OSI model approach. The interviewer is testing: Asymmetric Routing, stateful firewalls.. I structure my answer around systematic triage first, root cause analysis second, and permanent remediation third.""
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🛠️ Production Runbook & Step-by-Step Resolution
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Production Solution & Architecture
This is called Asymmetric Routing. The corporate firewall on ISP 2 is a stateful firewall. Stateful firewalls maintain an internal table of all outbound connections (the TCP handshake, sequence numbers, etc.). Because the initial outbound SYN packet left entirely through ISP 1's firewall, ISP 2’s firewall never saw the connection originate. When the SYN-ACK or data packets arrive on ISP 2, the firewall checks its state table, finds no existing outbound connection matching those IPs/ports, assumes the packet is a blind intrusion attempt, and rightfully drops it. *Fix:* Ensure BGP routing enforces symmetry, or dynamically share state tables between the two firewalls (HA clustering).
💡 The Senior SRE Gold Nugget (Key Architectural Takeaway)
"Pro-Tip: This is called Asymmetric Routing.."
⚡ 60-Second Elevator Pitch Talking Points
- Immediate Triage: This is called Asymmetric Routing.
- Run targeted verification commands before modifying configuration.
- Automate permanent guardrails (CI check, alerts, IaC policy) to prevent recurrence.
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