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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?
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.
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.
- ✗
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.
Go deeper
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
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.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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