The most likely cause is that LDP is not enabled on the interface toward 192.168.3.3. When LDP is disabled on the outgoing interface, the router cannot establish a label-switched path with the next-hop neighbor, so it never receives a label binding for the prefix 10.4.4.0/24. Without an incoming label-to-outgoing label mapping, the MPLS forwarding table shows 'Untagged' in the outgoing label column, meaning the router must pop the label and forward the packet as a standard IP packet. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of the MPLS label distribution process and the relationship between the LIB and LFIB. A common trap is assuming 'Untagged' indicates a misconfiguration of the local label itself, when in fact the issue is the missing LDP session on the egress interface. Memory tip: no LDP neighbor, no label swap—only a pop.
350-501 MPLS and Segment Routing Practice Question
This 350-501 practice question tests your understanding of mpls and segment routing. 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.
Exhibit
Refer to the exhibit.
PE1# show mpls forwarding-table
Local Outgoing Prefix Bytes Label Outgoing Next Hop
Label Label or Tunnel Id Switched interface
16 Pop Label 10.1.1.0/24 0 Gi0/0/0 192.168.1.1
17 18 10.2.2.0/24 0 Gi0/0/1 192.168.2.2
18 19 10.3.3.0/24 0 Gi0/0/1 192.168.2.2
19 Untagged 10.4.4.0/24 0 Gi0/0/2 192.168.3.3
Refer to the exhibit. A network engineer notices that the local label 19 for prefix 10.4.4.0/24 shows 'Untagged' in the outgoing label column. 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
✓
LDP is not enabled on the interface toward 192.168.3.3.
When LDP is not enabled on the interface toward the next-hop router (192.168.3.3), the local router cannot exchange label bindings with that neighbor. As a result, the local label for prefix 10.4.4.0/24 remains in the LIB but is not programmed into the LFIB with an outgoing label, showing 'Untagged' because the router must pop the label (or forward as IP) when sending traffic to that next hop.
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 next-hop router 192.168.3.3 is unreachable.
Why it's wrong here
If unreachable, the entry would not be in the LFIB.
✗
Penultimate Hop Popping is enabled on the next-hop router.
LDP is not enabled on the interface toward 192.168.3.3.
Why this is correct
If LDP is not enabled, no label is received from the next-hop, resulting in 'Untagged'.
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between 'Untagged' (no label from LDP neighbor) and 'Pop tag' (implicit-null from PHP), leading candidates to mistakenly attribute 'Untagged' to PHP when it actually indicates a missing LDP adjacency.
Trap categories for this question
Command / output trap
PHP would show 'Pop Label', not 'Untagged'.
Detailed technical explanation
How to think about this question
LDP uses UDP for discovery (hello messages on port 646) and TCP for session establishment and label exchange. If LDP is disabled on an interface (e.g., via 'no mpls ip' under the interface), the router will not form an LDP adjacency with the neighbor, so no label bindings are received for that next hop. In the LFIB, 'Untagged' indicates that the router must perform IP lookup (or impose a label based on a different LDP session) rather than swapping labels, which can lead to suboptimal forwarding or MPLS path failure in a full LSP.
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 at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
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.
MPLS and Segment Routing — This question tests MPLS and Segment Routing — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: LDP is not enabled on the interface toward 192.168.3.3. — When LDP is not enabled on the interface toward the next-hop router (192.168.3.3), the local router cannot exchange label bindings with that neighbor. As a result, the local label for prefix 10.4.4.0/24 remains in the LIB but is not programmed into the LFIB with an outgoing label, showing 'Untagged' because the router must pop the label (or forward as IP) when sending traffic to that next hop.
What should I do if I get this 350-501 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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
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. An engineer notices that MPLS VPN traffic is taking a suboptimal path because the MPLS label binding for the BGP next-hop loopback is missing. What is the most likely cause?
easy
A.The remote PE is configured with a different VPN ID.
B.The local PE does not have a route to its own loopback.
✓ C.LDP is not enabled on the core-facing interfaces.
D.LDP is not enabled on the PE-CE interface.
Why C: LDP must be enabled on core-facing interfaces to distribute labels for the loopback. If it is missing, no label is available.
Last reviewed: Jun 11, 2026
Question Discussion
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