Question 394 of 2,015

VRF and Path Isolation →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the BLUE VRF’s interface is missing the ip vrf forwarding BLUE command, leaving it in the global routing table. This is the most likely cause because when an interface lacks the ip vrf forwarding command, it does not belong to the VRF, so all traffic ingressing or egressing that interface uses the global routing table instead of the VRF’s routing table. Even though the BLUE VRF has a static default route and correct route-target import/export values (200:2), the interface itself cannot leverage that VRF table, leading to intermittent failures—especially when overlapping IPs exist between the RED and BLUE VRFs, as the global table may have conflicting or missing routes. On the ENCOR 350-401 exam, this scenario tests your understanding that VRF configuration is a two-step process: defining the VRF and then binding an interface to it via ip vrf forwarding. A common trap is assuming route-target configuration alone ensures VRF connectivity, but the interface binding is equally critical. Memory tip: “No VRF on the interface means no VRF in the fight”—always verify the interface is explicitly assigned to the VRF.

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 service provider uses MPLS L3VPN with multiple VRFs on a Cisco ASR 1000 PE router. One customer VRF (RED) has overlapping IP addresses with another VRF (BLUE). The engineer configures route-target import/export as 100:1 for RED and 200:2 for BLUE. Both VRFs have a static default route pointing to the CE. The PE receives VPNv4 routes from the route reflector for both VRFs. However, traffic from RED to its CE is working, but traffic from BLUE to its CE is intermittently failing. 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 1hardmultiple choice

Read the full MPLS explanation →

A
The BLUE VRF's interface is not configured with the ip vrf forwarding BLUE command, so the interface is in the global routing table.
Correct because if the interface is not associated with the VRF, traffic from that interface uses the global table, causing intermittent failures when the global table has conflicting routes.
B
The route-target import for BLUE is 200:2, but the route reflector exports routes with a different route-target.
Why wrong: Incorrect because if the route-target were mismatched, no VPNv4 routes would be imported into BLUE, causing complete failure, not intermittent.
C
The PE router has too many VRFs, causing memory exhaustion.
Why wrong: Incorrect because memory exhaustion would affect all VRFs, not just one.
D
The BLUE VRF is missing the rd command.
Why wrong: Incorrect because a missing route distinguisher would prevent VPNv4 route advertisement, causing complete failure, not intermittent.

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 BLUE VRF's interface is not configured with the ip vrf forwarding BLUE command, so the interface is in the global routing table.

The correct answer is A because if the BLUE VRF's interface is missing the 'ip vrf forwarding BLUE' command, the interface remains in the global routing table. This means traffic from the BLUE VRF will be forwarded using the global routing table instead of the VRF's routing table, causing intermittent failures when the global table does not have a route to the CE or when the CE's IP overlaps with another VRF's subnet. The static default route configured in the BLUE VRF would not be used, leading to connectivity issues.

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 BLUE VRF's interface is not configured with the ip vrf forwarding BLUE command, so the interface is in the global routing table.
Why this is correct
Correct because if the interface is not associated with the VRF, traffic from that interface uses the global table, causing intermittent failures when the global table has conflicting routes.
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 route-target import for BLUE is 200:2, but the route reflector exports routes with a different route-target.
Why it's wrong here
Incorrect because if the route-target were mismatched, no VPNv4 routes would be imported into BLUE, causing complete failure, not intermittent.
✗
The PE router has too many VRFs, causing memory exhaustion.
Why it's wrong here
Incorrect because memory exhaustion would affect all VRFs, not just one.
✗
The BLUE VRF is missing the rd command.
Why it's wrong here
Incorrect because a missing route distinguisher would prevent VPNv4 route advertisement, causing complete failure, not intermittent.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The trap here is that candidates often focus on route-target or RD mismatches as the cause of VRF connectivity issues, but Cisco tests the fundamental requirement that each VRF interface must be explicitly bound to the VRF using 'ip vrf forwarding', otherwise the VRF's routing table is not used for that interface.

Detailed technical explanation

How to think about this question

In MPLS L3VPN, the 'ip vrf forwarding <vrf-name>' command on an interface binds that interface to the VRF, ensuring that traffic received on that interface is looked up in the VRF's routing table and that traffic sourced from the VRF uses the correct forwarding table. Without this command, the interface remains in the global routing table, causing VRF traffic to be dropped or misrouted, especially when overlapping IPs exist between VRFs. This is a common misconfiguration that leads to asymmetric routing or blackholing, and it can be verified with 'show ip vrf interfaces' or 'show ip route vrf <vrf-name>'.

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 network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.

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 BLUE VRF's interface is not configured with the ip vrf forwarding BLUE command, so the interface is in the global routing table. — The correct answer is A because if the BLUE VRF's interface is missing the 'ip vrf forwarding BLUE' command, the interface remains in the global routing table. This means traffic from the BLUE VRF will be forwarded using the global routing table instead of the VRF's routing table, causing intermittent failures when the global table does not have a route to the CE or when the CE's IP overlaps with another VRF's subnet. The static default route configured in the BLUE VRF would not be used, leading to connectivity issues.

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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