hardMultiple Choice
300-410 Practice Question: Is troubleshooting MPLS traffic where packets are…
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?
⚠ Common exam trap
The trap is assuming that a valid label in the LFIB is sufficient for label switching — candidates overlook that the outgoing interface must also be MPLS-enabled for the data plane to forward labeled packets.
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.
For MPLS label switching to occur on an interface, the interface must have MPLS enabled with the 'mpls ip' command (or 'mpls ldp' depending on platform). If 'show ip cef' shows the outgoing interface is not MPLS-enabled, the router falls back to IP forwarding even though a label exists in the LFIB. Enabling MPLS on the interface restores label imposition and forwarding.
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
MPLS forwarding requires the outgoing interface to be MPLS-enabled. Without the mpls ip command, Cisco Express Forwarding shows that interface as non-MPLS, so labelled packets fall back to IP forwarding despite a valid label existing in the forwarding table.
- ✗
The CEF is disabled globally.
Why it's wrong here
With CEF disabled globally, no Cisco Express Forwarding adjacency exists at all, so the forwarding table could not show a valid label for the FEC as the stem states. It is tempting because CEF is a prerequisite for MPLS forwarding, but the symptom contradicts global disablement.
- ✗
The mpls label protocol ldp command is missing on the interface.
Why it's wrong here
A missing mpls label protocol ldp command disables LDP advertisement on the interface, but the stem already confirms a valid label exists in the forwarding table, so label distribution is working. It is tempting because LDP interface configuration is a common MPLS troubleshooting checkpoint.
- ✗
The interface is a loopback interface.
Why it's wrong here
A loopback is a logical interface and cannot carry MPLS label-switched traffic; CEF would still show it as not MPLS-enabled, but the stem describes a forwarding path, not a local host route. It is tempting because loopbacks commonly appear in MPLS router-ID and LDP router-ID configuration.
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Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
Key term
MPLS Label Distribution
MPLS Label Distribution is the process by which routers exchange labels that tell them how to forward packets across a network without looking at the IP address each time.
Key term
LDP Protocol
LDP, or Label Distribution Protocol, is a protocol that routers use to automatically exchange labels that enable MPLS (Multiprotocol Label Switching) to create fast, efficient paths for data packets across a network.
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Same concept, more angles
1 more way this is tested on 300-410
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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: In MPLS, an 'Untagged' outgoing label in the LFIB indicates that the router will pop the label and forward the packet as unlabeled (IP) or that the outgoing interface is not enabled for MPLS/LDP. Since the IGP and LDP neighbors are fine, the most likely cause is that LDP is not enabled on the outgoing interface, so no label is advertised or used for that FEC. This results in the router forwarding the packet without a label, which can cause drops if the next hop expects a label.
JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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