- A
Increase the number of tunnels to four to distribute traffic better.
Why wrong: More tunnels can increase complexity and still have ECMP issues.
- B
Change from static routing to dynamic routing (BGP) to allow the Cloud VPN gateway to advertise routes and avoid ECMP issues.
BGP provides better control over path selection and is recommended for HA VPN with multiple tunnels.
- C
Set the on-premises router to active-passive mode using only one tunnel at a time.
Why wrong: While this would avoid ECMP issues, it reduces redundancy and throughput, and is not the best practice.
- D
Configure traffic selector policies on the VPN tunnels to ensure each tunnel only handles specific subnets.
Why wrong: Traffic selectors are for IKE, but do not fix ECMP issues with static routing.
HA VPN Static Routing — ECMP Intermittent Traffic Issue
This PCNE practice question tests your understanding of implementing a virtual private cloud. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A company is migrating from an on-premises data center to Google Cloud. They have set up a High-Availability VPN (with two tunnels) between their on-premises router and a Cloud VPN gateway in a VPC. They use static routing. The on-premises network uses RFC 1918 addresses (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) and the VPC uses 10.1.0.0/16. They have configured static routes in the VPC for the on-premises ranges pointing to the VPN gateway. However, they notice that traffic from on-premises to the VPC is intermittent: sometimes packets go through tunnel 1, sometimes through tunnel 2, and sometimes they drop. The on-premises router is configured to use both tunnels in an active-active mode with equal-cost multipath (ECMP). What is the most likely cause of the intermittent drops?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Quick Answer
The answer is to switch from static routing to dynamic routing with BGP, as this resolves the ECMP intermittent traffic issue by enabling proper route advertisement and path selection. When using static routing with an HA VPN in active-active mode, the on-premises router’s ECMP distribution across two tunnels can cause packet reordering or drops because the Cloud VPN gateway does not coordinate per-flow hashing for static routes, leading to inconsistent forwarding and intermittent connectivity. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of Cloud VPN limitations with static routing versus BGP’s ability to establish a single, stable next-hop via the gateway’s dynamic route exchange, a common trap where candidates overlook that static routes treat each tunnel independently. Remember: static ECMP drops, BGP stops the flops.
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
Change from static routing to dynamic routing (BGP) to allow the Cloud VPN gateway to advertise routes and avoid ECMP issues.
The most likely cause of intermittent drops is that the on-premises router is using ECMP to distribute traffic across both VPN tunnels, but Cloud VPN with static routing does not support ECMP for incoming traffic. This can lead to out-of-order packets or drops for the same flow if packets arrive on different tunnels. Switching to dynamic routing with BGP allows the Cloud VPN gateway to establish BGP sessions over each tunnel, which enables proper route advertisement and ECMP support by creating a single logical interface. BGP routing also allows the gateway to better handle path selection and failover, thus resolving the intermittent drops. Therefore, option B is correct.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Increase the number of tunnels to four to distribute traffic better.
Why it's wrong here
More tunnels can increase complexity and still have ECMP issues.
- ✓
Change from static routing to dynamic routing (BGP) to allow the Cloud VPN gateway to advertise routes and avoid ECMP issues.
- ✗
Set the on-premises router to active-passive mode using only one tunnel at a time.
Why it's wrong here
While this would avoid ECMP issues, it reduces redundancy and throughput, and is not the best practice.
- ✗
Configure traffic selector policies on the VPN tunnels to ensure each tunnel only handles specific subnets.
Why it's wrong here
Traffic selectors are for IKE, but do not fix ECMP issues with static routing.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
- CIDR notation defines the prefix length.
- Block size helps identify subnet boundaries.
- Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
- The required host count determines the smallest suitable subnet.
TExam Day Tips
- Write the block size before choosing the subnet.
- Check whether the question asks for hosts, subnets or a specific address range.
- Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.
Visual reference
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related PCNE subnetting questions on CIDR, address ranges, and subnet selection.
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FAQ
Questions learners often ask
What does this PCNE question test?
Implementing a Virtual Private Cloud — This question tests Implementing a Virtual Private Cloud — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Change from static routing to dynamic routing (BGP) to allow the Cloud VPN gateway to advertise routes and avoid ECMP issues. — The most likely cause of intermittent drops is that the on-premises router is using ECMP to distribute traffic across both VPN tunnels, but Cloud VPN with static routing does not support ECMP for incoming traffic. This can lead to out-of-order packets or drops for the same flow if packets arrive on different tunnels. Switching to dynamic routing with BGP allows the Cloud VPN gateway to establish BGP sessions over each tunnel, which enables proper route advertisement and ECMP support by creating a single logical interface. BGP routing also allows the gateway to better handle path selection and failover, thus resolving the intermittent drops. Therefore, option B is correct.
What should I do if I get this PCNE question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related PCNE subnetting questions on CIDR, address ranges, and subnet selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Last reviewed: Jun 24, 2026
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.
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