Question 1,877 of 2,152

MPLS Operations →hardMultiple ChoiceObjective-mapped

MPLS Forwarding Without Label — Interface Not MPLS-Enabled | Cisco CCNP ENARSI 300-410 Explained

This 300-410 practice question tests your understanding of mpls operations. 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 troubleshooting MPLS traffic where packets are being forwarded without a label (IP forwarding) instead of being label-switched. The engineer runs show mpls forwarding-table and sees that the FEC for the destination prefix has a valid label. However, show ip cef shows that the outgoing interface is not MPLS-enabled. 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.

A
The outgoing interface is missing the mpls ip command.
Correct because mpls ip on the interface enables MPLS forwarding; without it, CEF will not impose a label and will forward the packet as IP.
B
The CEF is disabled globally.
Why wrong: Incorrect because CEF must be enabled for MPLS; if CEF were disabled, show ip cef would not work, and the symptom would be different.
C
The mpls label protocol ldp command is missing on the interface.
Why wrong: Incorrect because LDP is already assigning labels; the mpls label protocol command only affects label distribution, not forwarding.
D
The interface is a loopback interface.
Why wrong: Incorrect because loopback interfaces are not typically used for MPLS forwarding; the issue is with a physical or subinterface.

Full breakdown with real-world context →

Quick Answer

The answer is that the outgoing interface is missing the **mpls ip** command, which prevents label imposition even when a valid label exists in the LFIB. This occurs because MPLS forwarding requires the interface to be explicitly enabled for MPLS; without that command, CEF will forward packets using IP routing instead of imposing the label, regardless of what LDP has assigned. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of the separation between label distribution and label imposition—a common trap is assuming that a label in the LFIB guarantees MPLS forwarding, but the interface-level **mpls ip** is the gatekeeper. Remember the memory tip: **"Label in the table, but no label on the wire—check the interface for mpls ip fire."**

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 outgoing interface is missing the mpls ip command.

The LFIB has a label, but CEF is not imposing it, indicating that the outgoing interface is not configured for MPLS forwarding. Even if LDP assigns a label, the interface must have mpls ip enabled for label imposition to occur.

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 outgoing interface is missing the mpls ip command.
Why this is correct
Correct because mpls ip on the interface enables MPLS forwarding; without it, CEF will not impose a label and will forward the packet as IP.
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 CEF is disabled globally.
Why it's wrong here
Incorrect because CEF must be enabled for MPLS; if CEF were disabled, show ip cef would not work, and the symptom would be different.
✗
The mpls label protocol ldp command is missing on the interface.
Why it's wrong here
Incorrect because LDP is already assigning labels; the mpls label protocol command only affects label distribution, not forwarding.
✗
The interface is a loopback interface.
Why it's wrong here
Incorrect because loopback interfaces are not typically used for MPLS forwarding; the issue is with a physical or subinterface.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.

Trap categories for this question

Command / output trap
Incorrect because CEF must be enabled for MPLS; if CEF were disabled, show ip cef would not work, and the symptom would be different.

Detailed technical explanation

How to think about this question

This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.

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.
Use explanations to understand the rule behind the answer.

TExam Day Tips

Underline the problem statement mentally.
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 300-410 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 300-410 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.

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FAQ

Questions learners often ask

What does this 300-410 question test?

MPLS Operations — This question tests MPLS Operations — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The outgoing interface is missing the mpls ip command. — The LFIB has a label, but CEF is not imposing it, indicating that the outgoing interface is not configured for MPLS forwarding. Even if LDP assigns a label, the interface must have mpls ip enabled for label imposition to occur.

What should I do if I get this 300-410 question wrong?

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.

About these practice questions

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Same concept, more angles

1 more ways this is tested on 300-410

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. A network engineer is troubleshooting MPLS traffic where packets are being dropped at a P router. The engineer runs show mpls forwarding-table and sees that the outgoing label for a specific FEC is 'Untagged' instead of a valid label. The IGP is running correctly, and LDP neighbors are established. What is the most likely cause?

medium

✓ A.LDP is not enabled on the outgoing interface.
B.The IGP metric is too high, causing LDP to prefer a different path.
C.The mpls label range is exhausted.
D.The router is configured with mpls ldp advertise-labels for host routes only.

Why A: The 'Untagged' label in the LFIB indicates that LDP has not assigned a label for that FEC on the outgoing interface. This typically happens when LDP is not enabled on the outgoing interface, either because the interface is not configured with mpls ip or because the interface is passive under LDP.

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