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."**
300-410 MPLS Operations Practice Question
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.
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: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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
Static NAT maps one inside address to one outside address.
✗
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: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
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
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
MPLS Operations — This question tests MPLS Operations — Static NAT maps one inside address to one outside address..
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?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
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?
Static NAT maps one inside address to one outside address.
About these practice questions
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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.
Last reviewed: Jun 18, 2026
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