PCNE Implementing VPC Instances Practice Question
An engineer is troubleshooting connectivity from a Compute Engine instance (internal IP: 10.0.0.2) to an on-premises server (IP: 203.0.113.5) over a Cloud VPN tunnel. The traffic reaches the on-premises network, but the return traffic is dropped. What is the most likely cause?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The on-premises router is not advertising the return route via BGP, causing asymmetric routing.
For asymmetric routing, GCP may drop return traffic if it does not match the forwarding route. Cloud VPN supports dynamic routing (BGP) which advertises the on-premises prefix. However, if the on-premises router forwards return traffic through a different path (e.g., internet), GCP's firewall drops it because the return packet does not come via the VPN tunnel (asymmetric).
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The Cloud VPN tunnel is down.
Why it's wrong here
If the tunnel were down, outbound traffic would not reach on-prem.
- ✗
The MTU is set to 1500 on the VPN tunnel.
Why it's wrong here
MTU 1500 is default; GCP supports up to 1460 for VM instances, but that would cause fragmentation, not dropping.
- ✗
The instance's firewall is blocking ICMP.
Why it's wrong here
Connectivity is likely tested with TCP/UDP; ICMP blocking is not the most likely cause.
- ✓
The on-premises router is not advertising the return route via BGP, causing asymmetric routing.
Why this is correct
Return traffic may take a different path, which GCP drops because it doesn't match the expected ingress interface.
Quick reference
Asymmetric Encryption Algorithm Comparison
| Algorithm | Key Exchange | Signatures | Equivalent Security Key | Notes |
|---|---|---|---|---|
| RSA-3072 | Yes | Yes | 128-bit | Widely deployed; slow for bulk data |
| ECDSA P-256 | No | Yes | 128-bit | Fast signatures; standard TLS certs |
| ECDH / ECDHE | Yes | No | 128-bit | Perfect forward secrecy in TLS 1.3 |
| DH / DHE | Yes | No | 128-bit (3072-bit key) | Replaced by ECDHE in modern TLS |
| Ed25519 | No | Yes | ~128-bit | SSH keys, modern PKI |
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This PCNE practice question is part of Courseiva's free Google Cloud certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PCNE exam.