Question 483 of 2,015

VRF and Path Isolation →mediumMultiple ChoiceObjective-mapped

350-401 VRF and Path Isolation Practice Question

This 350-401 practice question tests your understanding of vrf and path isolation. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 network engineer is configuring MPLS L3VPN on a Cisco IOS-XE PE router. The engineer creates a VRF named CUSTOMER_A with route-target import and export 100:1. After configuring the VRF on the interface connected to the CE router, the CE router can ping the PE's VRF interface IP, but cannot reach any remote VPNv4 routes. The BGP session between PE and route reflector is up. What is the most likely cause?

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.

Question 1mediummultiple choice

Open the full BGP breakdown →

A
The route-target import/export values are mismatched with the route reflector's configuration.
Why wrong: Incorrect because the route-target values are only relevant for route distribution among PEs; the route reflector simply reflects VPNv4 routes regardless of RT.
B
The VRF is not activated under BGP using the address-family ipv4 vrf CUSTOMER_A command.
Correct because without this command, the PE does not redistribute VRF routes into VPNv4 or import VPNv4 routes into the VRF.
C
The CE router is not configured with a default route pointing to the PE.
Why wrong: Incorrect because the CE can ping the PE's VRF interface, indicating Layer 3 connectivity; the issue is with remote route reachability.
D
The PE router needs the mpls ip command on the interface facing the CE router.
Why wrong: Incorrect because MPLS is not required on the CE-facing interface; it is needed on core-facing interfaces.

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 VRF is not activated under BGP using the address-family ipv4 vrf CUSTOMER_A command.

The CE router can ping the PE's VRF interface IP, confirming Layer 2 and VRF interface configuration are correct. However, the CE cannot reach remote VPNv4 routes, which indicates that the PE is not advertising or installing those routes into the VRF. The most likely cause is that the VRF CUSTOMER_A has not been activated under BGP using the 'address-family ipv4 vrf CUSTOMER_A' command, which is required to exchange IPv4 routes between the PE and CE within the VRF context and to redistribute them into MP-BGP for VPNv4 propagation.

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 route-target import/export values are mismatched with the route reflector's configuration.
Why it's wrong here
Incorrect because the route-target values are only relevant for route distribution among PEs; the route reflector simply reflects VPNv4 routes regardless of RT.
✓
The VRF is not activated under BGP using the address-family ipv4 vrf CUSTOMER_A command.
Why this is correct
Correct because without this command, the PE does not redistribute VRF routes into VPNv4 or import VPNv4 routes into the VRF.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The CE router is not configured with a default route pointing to the PE.
Why it's wrong here
Incorrect because the CE can ping the PE's VRF interface, indicating Layer 3 connectivity; the issue is with remote route reachability.
✗
The PE router needs the mpls ip command on the interface facing the CE router.
Why it's wrong here
Incorrect because MPLS is not required on the CE-facing interface; it is needed on core-facing interfaces.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the misconception that a working BGP session to the route reflector and correct route-target values alone are sufficient for VPNv4 route exchange, when in fact the VRF must be explicitly activated under BGP to enable route advertisement and import.

Detailed technical explanation

How to think about this question

In MPLS L3VPN, the PE must activate the VRF under BGP using 'address-family ipv4 vrf <name>' to enable the exchange of IPv4 routes with the CE (via eBGP or static/redistribution) and to inject those routes into the VPNv4 address family with the appropriate route-target. Without this activation, the VRF routes are not advertised to the route reflector, and remote VPNv4 routes are not imported into the VRF, even if the BGP session to the route reflector is up. A common real-world scenario is forgetting this step after creating the VRF and assigning it to an interface, leading to isolated VRF connectivity.

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 practitioner preparing for the 350-401 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.

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 350-401 question test?

VRF and Path Isolation — This question tests VRF and Path Isolation — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The VRF is not activated under BGP using the address-family ipv4 vrf CUSTOMER_A command. — The CE router can ping the PE's VRF interface IP, confirming Layer 2 and VRF interface configuration are correct. However, the CE cannot reach remote VPNv4 routes, which indicates that the PE is not advertising or installing those routes into the VRF. The most likely cause is that the VRF CUSTOMER_A has not been activated under BGP using the 'address-family ipv4 vrf CUSTOMER_A' command, which is required to exchange IPv4 routes between the PE and CE within the VRF context and to redistribute them into MP-BGP for VPNv4 propagation.

What should I do if I get this 350-401 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: "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?

Read the scenario before looking for a memorised answer.

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