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Question 723 of 1,000

Implementing a Virtual Private Cloud →mediumMultiple SelectObjective-mapped

Multi-Tier VPC Firewall Rules Using Tags

This PCNE practice question tests your understanding of implementing a virtual private cloud. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 has a VPC with three tiers: web, app, and db. They want to enforce that only the web tier can communicate with the app tier on TCP port 8080, and only the app tier can communicate with the db tier on TCP port 3306. All instances are in the same region but different subnets. Which TWO firewall rules should be created? (Choose 2.)

A
An ingress rule on the db tier instances with tag 'db' allowing TCP 3306 from instances with tag 'app'.
This ensures only app tier can reach db tier on port 3306.
B
An egress rule on the web instances with tag 'web' allowing TCP 8080 to the app subnet.
Why wrong: Egress rules control outbound traffic, but the requirement is to control ingress into app and db tiers; ingress rules are more direct.
C
An egress rule on the web subnet allowing TCP 8080 to any destination.
Why wrong: This does not restrict which tier can communicate; it allows all egress on port 8080, not just to app.
D
An ingress rule on the app tier instances with tag 'app' allowing TCP 8080 from instances with tag 'web'.
This ensures only web tier can reach app tier on port 8080.
E
An ingress rule on the web subnet allowing TCP 8080 from the app subnet.
Why wrong: This allows app to communicate with web on 8080, which is the opposite direction.

Full breakdown with real-world context →

Quick Answer

The correct answer is to create an ingress rule on the app tier instances with tag 'app' allowing TCP 8080 from instances with tag 'web', and an ingress rule on the db tier instances with tag 'db' allowing TCP 3306 from instances with tag 'app'. This approach works because Google Cloud firewall rules are stateful and evaluated at the instance level using source and destination tags, meaning you control access by defining what traffic is allowed to enter the target tier rather than by restricting outbound traffic from the source. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of multi-tier VPC firewall rules using tags, a common pattern for microsegmentation where each tier only listens for traffic from the preceding tier. A frequent trap is choosing egress rules on the source tier, which fail to block other unauthorized sources from reaching the destination; always remember that ingress rules on the destination are the correct enforcement point. Memory tip: “Ingress on the listener, tag the source” — the rule goes on the tier receiving traffic, and the source tag identifies who is allowed to knock.

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

✓

An ingress rule on the db tier instances with tag 'db' allowing TCP 3306 from instances with tag 'app'.

To control traffic between tiers, ingress firewall rules should be applied to the destination instances using target tags. For web-to-app (TCP 8080), place an ingress rule on app instances (tag 'app') allowing TCP 8080 from source tag 'web' (Option D). For app-to-db (TCP 3306), place an ingress rule on db instances (tag 'db') allowing TCP 3306 from source tag 'app' (Option A). Option B is an egress rule on web, which controls outbound traffic but does not prevent other sources from reaching app. Option C allows traffic to any destination, violating the principle of least privilege. Option E incorrectly places an ingress rule on the web subnet, but the web tier should be the source, not the destination. Therefore, the correct answers are A and D.

Key principle: Authentication proves identity; authorization controls what that identity can do after login. Both must work for full privileged access.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✓
An ingress rule on the db tier instances with tag 'db' allowing TCP 3306 from instances with tag 'app'.
Why this is correct
This ensures only app tier can reach db tier on port 3306.
Related concept
Authentication checks who the user is.
✗
An egress rule on the web instances with tag 'web' allowing TCP 8080 to the app subnet.
Why it's wrong here
Egress rules control outbound traffic, but the requirement is to control ingress into app and db tiers; ingress rules are more direct.
✗
An egress rule on the web subnet allowing TCP 8080 to any destination.
Why it's wrong here
This does not restrict which tier can communicate; it allows all egress on port 8080, not just to app.
✓
An ingress rule on the app tier instances with tag 'app' allowing TCP 8080 from instances with tag 'web'.
Why this is correct
This ensures only web tier can reach app tier on port 8080.
Related concept
Authentication checks who the user is.
✗
An ingress rule on the web subnet allowing TCP 8080 from the app subnet.
Why it's wrong here
This allows app to communicate with web on 8080, which is the opposite direction.

Common exam traps

Common exam trap: authentication is not authorization

Logging in proves the user can authenticate. It does not automatically mean the user is allowed to enter privileged or configuration mode. Watch for AAA authorization, privilege level and command authorization details.

Detailed technical explanation

How to think about this question

This kind of question is testing the difference between identity and permission. A user may successfully log in to a router because authentication is working, but still fail to enter configuration mode because authorization is missing, misconfigured or mapped to a lower privilege level.

KKey Concepts to Remember

Authentication checks who the user is.
Authorization controls what the user is allowed to do after login.
Privilege levels affect access to EXEC and configuration commands.
AAA, TACACS+ and RADIUS can separate login success from command access.

TExam Day Tips

Do not assume successful login means full administrative access.
Look for words such as cannot enter configuration mode, privilege level, authorization or command access.
Separate login problems from permission problems before choosing the answer.

Key takeaway

Authentication proves identity; authorization controls what that identity can do after login. Both must work for full privileged access.

Real-world example

How this comes up in practice

A media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.

Visual reference

What to study next

Got this wrong? Here's your next step.

Review Cisco AAA concepts — authentication, authorization, and accounting. Study privilege levels (0–15), command authorization under TACACS+, and how RADIUS differs. Then practise related PCNE questions on access control and AAA configuration.

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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 — Authentication checks who the user is..

What is the correct answer to this question?

The correct answer is: An ingress rule on the db tier instances with tag 'db' allowing TCP 3306 from instances with tag 'app'. — To control traffic between tiers, ingress firewall rules should be applied to the destination instances using target tags. For web-to-app (TCP 8080), place an ingress rule on app instances (tag 'app') allowing TCP 8080 from source tag 'web' (Option D). For app-to-db (TCP 3306), place an ingress rule on db instances (tag 'db') allowing TCP 3306 from source tag 'app' (Option A). Option B is an egress rule on web, which controls outbound traffic but does not prevent other sources from reaching app. Option C allows traffic to any destination, violating the principle of least privilege. Option E incorrectly places an ingress rule on the web subnet, but the web tier should be the source, not the destination. Therefore, the correct answers are A and D.

What should I do if I get this PCNE question wrong?

What is the key concept behind this question?

Authentication checks who the user is.

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