The answer is to ensure on-premises routers advertise the prefix with the same AS_PATH length and MED. This configuration enables BGP ECMP on Cloud Router for load balancing because Cloud Router only considers paths as equal-cost when both the AS_PATH length and the Multi-Exit Discriminator (MED) values are identical. When these attributes match, Cloud Router installs multiple routes with equal preference, allowing traffic to be distributed across both Dedicated Interconnects while also providing automatic failover if one path fails. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding that ECMP is not automatic with BGP—it requires explicit attribute parity, and a common trap is assuming different AS_PATH lengths can still be load-balanced. Remember the key condition: for ECMP to work, both the AS_PATH and MED must match. A useful memory tip is "Same Path, Same MED—Load Balance Ahead."
PCNE Implementing hybrid interconnectivity Practice Question
This PCNE practice question tests your understanding of implementing hybrid interconnectivity. 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 global company has multiple on-premises data centers connected to Google Cloud via separate Dedicated Interconnects. Each on-premises site advertises the same IP prefix for a critical application. They want to ensure that traffic from Google Cloud to that prefix is load-balanced across the two interconnects and also provide automatic failover. Which configuration on Cloud Router meets this requirement?
Configure Cloud Router with the same MED value for both paths
Why wrong: MED is set on the received routes; Cloud Router does not set MED on received routes. The on-premises routers must advertise with the same MED.
B
Use BGP multipath on Cloud Router with 'maximum-paths' set to 2
Why wrong: Cloud Router does not allow setting 'maximum-paths' as it automatically load balances if paths are equal.
C
Ensure on-premises routers advertise the prefix with the same AS_PATH length and MED
ECMP requires equal BGP path attributes including AS_PATH length and MED.
D
Enable 'set-community' on the on-premises routers to mark routes equally
Why wrong: Community tags do not cause ECMP; they are for policy.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Ensure on-premises routers advertise the prefix with the same AS_PATH length and MED
To load-balance and provide failover, you need equal-cost multi-path (ECMP) routing. Cloud Router supports ECMP only when the routes have the same MED and AS_PATH length. Setting both on-premises routers to advertise with the same attributes allows ECMP.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Configure Cloud Router with the same MED value for both paths
Why it's wrong here
MED is set on the received routes; Cloud Router does not set MED on received routes. The on-premises routers must advertise with the same MED.
✗
Use BGP multipath on Cloud Router with 'maximum-paths' set to 2
Why it's wrong here
Cloud Router does not allow setting 'maximum-paths' as it automatically load balances if paths are equal.
✓
Ensure on-premises routers advertise the prefix with the same AS_PATH length and MED
Why this is correct
ECMP requires equal BGP path attributes including AS_PATH length and MED.
Related concept
Static NAT maps one inside address to one outside address.
✗
Enable 'set-community' on the on-premises routers to mark routes equally
Why it's wrong here
Community tags do not cause ECMP; they are for policy.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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.
What to study next
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related PCNE NAT questions on configuration and troubleshooting.
Implementing hybrid interconnectivity — This question tests Implementing hybrid interconnectivity — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: Ensure on-premises routers advertise the prefix with the same AS_PATH length and MED — To load-balance and provide failover, you need equal-cost multi-path (ECMP) routing. Cloud Router supports ECMP only when the routes have the same MED and AS_PATH length. Setting both on-premises routers to advertise with the same attributes allows ECMP.
What should I do if I get this PCNE question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related PCNE NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. An organization has two Cloud VPN tunnels from the same on-premises router to a Cloud Router in Google Cloud. Both tunnels are using BGP, and the on-premises router is sending the same routes over both tunnels. The Cloud Router is configured to use 'route propagation' from a VPC network. Which of the following is true regarding route priority?
easy
✓ A.Cloud Router will use both tunnels for load balancing (ECMP) if the routes are identical
B.The tunnel with the lower local preference value will be used
C.The tunnel with the higher BGP MED value will be preferred
D.Only the tunnel with the lowest IP address will be used
Why A: When both BGP routes are identical in prefix, AS path, local preference, MED, and other attributes, Cloud Router uses ECMP (Equal-Cost Multi-Path) to load balance traffic across both Cloud VPN tunnels. Route propagation in the VPC network does not alter the BGP best-path selection; it simply injects the learned routes into the VPC routing table. Since the on-premises router advertises the same routes over both tunnels, Cloud Router treats them as equal-cost paths and distributes traffic across both tunnels.
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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.
Question Discussion
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