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Implementing hybrid interconnectivity →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the on-premises router is advertising the route with different MED values, which prevents unequal traffic load balancing on Cloud Router. BGP uses the Multi-Exit Discriminator (MED) attribute to influence inbound traffic from a neighboring AS; when two peers advertise the same prefix with different MEDs, the Cloud Router selects only the path with the lower MED, directing all traffic through that single peer. This concept tests your understanding of BGP path selection and the specific behavior of Google Cloud Router, which does not perform ECMP unless all BGP attributes—including MED—are identical. A common trap on the Google Professional Cloud Network Engineer exam is assuming that multiple BGP peers automatically create load balancing, but MED differentiation explicitly overrides that. For a memory tip, remember: “MED decides, so traffic divides—only when equal, paths will settle.”

PCNE Implementing hybrid interconnectivity Practice Question

This PCNE practice question tests your understanding of implementing hybrid interconnectivity. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.

Exhibit

Refer to the exhibit.

```
$ gcloud compute routers get-status my-router --region us-central1
...
result:
  bgpStatus:
    - peer: 10.0.0.1
      status: established
      learnedRoutes:
        - prefix: 10.1.0.0/16
          nextHop: 10.0.0.1
      advertisedRoutes:
        - prefix: 10.2.0.0/16
    - peer: 10.0.0.2
      status: established
      learnedRoutes:
        - prefix: 10.1.0.0/16
          nextHop: 10.0.0.2
      advertisedRoutes:
        - prefix: 10.2.0.0/16
```

A network engineer sees the above output from a Cloud Router. There are two BGP peers from the on-premises router (10.0.0.1 and 10.0.0.2). Both learned the same route 10.1.0.0/16 from their respective peers. However, traffic from Google Cloud to 10.1.0.0/16 is only going through the first peer (10.0.0.1) and not load-balanced. What could be the reason?

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.

Question 1hardmultiple choice

Open the full BGP breakdown →

Exhibit

Refer to the exhibit.

```
$ gcloud compute routers get-status my-router --region us-central1
...
result:
  bgpStatus:
    - peer: 10.0.0.1
      status: established
      learnedRoutes:
        - prefix: 10.1.0.0/16
          nextHop: 10.0.0.1
      advertisedRoutes:
        - prefix: 10.2.0.0/16
    - peer: 10.0.0.2
      status: established
      learnedRoutes:
        - prefix: 10.1.0.0/16
          nextHop: 10.0.0.2
      advertisedRoutes:
        - prefix: 10.2.0.0/16
```

A
The on-premises router is advertising the route with different MED values
Different MED values prevent ECMP; Cloud Router selects the route with the lower MED.
B
The on-premises router is advertising the route with different AS_PATH lengths
Why wrong: AS_PATH length difference would cause a longer path to be less preferred, but they could still be equal if lengths are same. The exhibit does not show AS_PATH.
C
The VPC routing mode is set to 'global'
Why wrong: Routing mode does not affect ECMP within a region.
D
Cloud Router has a limit of one route per prefix
Why wrong: Cloud Router can handle multiple routes per prefix and load-balances if attributes are equal.

Full breakdown with real-world context →

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 advertising the route with different MED values

Option A is correct because BGP uses the MED (Multi-Exit Discriminator) attribute to influence inbound traffic from a neighboring AS. When the on-premises router advertises the same route (10.1.0.0/16) to the Cloud Router via two peers (10.0.0.1 and 10.0.0.2) with different MED values, the Cloud Router will prefer the route with the lower MED value. This causes all traffic to be sent through the peer with the lower MED, preventing load balancing. By default, Cloud Router does not perform ECMP (Equal-Cost Multi-Path) for BGP routes unless the paths are identical in all BGP path selection criteria, including MED.

Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

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 on-premises router is advertising the route with different MED values
Why this is correct
Different MED values prevent ECMP; Cloud Router selects the route with the lower MED.
Clue confirmation
The clue word "first" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The on-premises router is advertising the route with different AS_PATH lengths
Why it's wrong here
AS_PATH length difference would cause a longer path to be less preferred, but they could still be equal if lengths are same. The exhibit does not show AS_PATH.
✗
The VPC routing mode is set to 'global'
Why it's wrong here
Routing mode does not affect ECMP within a region.
✗
Cloud Router has a limit of one route per prefix
Why it's wrong here
Cloud Router can handle multiple routes per prefix and load-balances if attributes are equal.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The trap here is that candidates often assume BGP automatically load-balances across multiple peers for the same prefix, forgetting that BGP's path selection algorithm picks a single best path unless all attributes (including MED) are equal, and that MED is compared even when AS_PATH lengths are the same.

Trap categories for this question

Command / output trap
AS_PATH length difference would cause a longer path to be less preferred, but they could still be equal if lengths are same. The exhibit does not show AS_PATH.

Detailed technical explanation

How to think about this question

In BGP, the MED attribute is a 4-byte value (RFC 4456) that is used to influence the entry point into an AS; it is compared only when the routes come from the same neighboring AS. Cloud Router uses standard BGP path selection, which compares MED after AS_PATH length and origin type. For load balancing to occur, the routes must be identical in all BGP attributes (weight, local preference, AS_PATH, origin, MED, IGP metric, etc.), and the Cloud Router must have ECMP enabled (which it does by default for equal-cost BGP routes). A common real-world scenario is when an on-premises router uses different MED values to steer traffic to specific links for traffic engineering, inadvertently breaking load balancing.

KKey Concepts to Remember

Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.

TExam Day Tips

Watch for words such as best, first, most likely and least administrative effort.
Review why wrong options are wrong, not only why the correct option is correct.

Key takeaway

Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

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.

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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FAQ

Questions learners often ask

What does this PCNE question test?

Implementing hybrid interconnectivity — This question tests Implementing hybrid interconnectivity — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The on-premises router is advertising the route with different MED values — Option A is correct because BGP uses the MED (Multi-Exit Discriminator) attribute to influence inbound traffic from a neighboring AS. When the on-premises router advertises the same route (10.1.0.0/16) to the Cloud Router via two peers (10.0.0.1 and 10.0.0.2) with different MED values, the Cloud Router will prefer the route with the lower MED value. This causes all traffic to be sent through the peer with the lower MED, preventing load balancing. By default, Cloud Router does not perform ECMP (Equal-Cost Multi-Path) for BGP routes unless the paths are identical in all BGP path selection criteria, including MED.

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

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

Are there clue words in this question I should notice?

Yes — watch for: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.

What is the key concept behind this question?

Read the scenario before looking for a memorised answer.

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Last reviewed: Jun 25, 2026

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