The answer is to enable 'mpls ldp advertise-labels' on the router. This command is correct because MPLS LDP sessions can be fully established between peers, but if label advertisement is administratively disabled—often through an access-list or a global override—the routers will not exchange label bindings for any FECs, leaving the LFIB empty despite a healthy session. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding that LDP session state and label distribution are separate processes; a common trap is assuming an up session guarantees label exchange. The exam may present a configuration where 'no mpls ldp advertise-labels' is applied, or where an explicit label advertisement policy blocks all prefixes. Remember the memory tip: “Session up, labels missing? Check if advertisement is listening.”
350-501 Networking Practice Question
This 350-501 practice question tests your understanding of networking. 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 LSP connectivity. The MPLS LDP session is up on both endpoints, but some MPLS labels are missing in the LFIB. Which configuration change would most likely resolve the issue?
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
✓
Enable 'mpls ldp advertise-labels'
Option B is correct because the 'mpls ldp advertise-labels' command ensures that LDP advertises label bindings to its peers. If this is disabled (default is enabled, but can be overridden), LDP sessions may be up but no labels are distributed, resulting in missing labels in the LFIB. Re-enabling it forces LDP to send label mappings for all FECs, resolving the connectivity issue.
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.
✗
Configure 'label protocol ldp' on all routers
Why it's wrong here
LDP is usually the default label protocol.
✓
Enable 'mpls ldp advertise-labels'
Why this is correct
This command ensures that LDP advertises labels for all FECs.
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.
✗
Configure 'mpls ip' on all interfaces
Why it's wrong here
Missing labels are not typically caused by missing 'mpls ip' if LDP is up.
✗
Enable 'mpls ldp explicit-null' on the ingress
Why it's wrong here
Explicit-null is for penultimate hop popping, not for missing labels.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between LDP session establishment (which can be up) and label advertisement (which can be suppressed via filters), leading candidates to overlook the 'advertise-labels' command and instead focus on interface-level MPLS enablement or protocol selection.
Detailed technical explanation
How to think about this question
LDP label advertisement is controlled by the 'mpls ldp advertise-labels' command, which can be configured with access-lists to selectively advertise labels. By default, LDP advertises all labels, but if a label advertisement filter is applied (e.g., via 'mpls ldp advertise-labels for acl'), labels for certain prefixes may be withheld, causing incomplete LFIB entries. This is a common troubleshooting scenario where sessions are up but label bindings are missing due to misconfigured advertisement policies.
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 practitioner preparing for the 350-501 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 exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Networking — This question tests Networking — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Enable 'mpls ldp advertise-labels' — Option B is correct because the 'mpls ldp advertise-labels' command ensures that LDP advertises label bindings to its peers. If this is disabled (default is enabled, but can be overridden), LDP sessions may be up but no labels are distributed, resulting in missing labels in the LFIB. Re-enabling it forces LDP to send label mappings for all FECs, resolving the connectivity issue.
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. Refer to the exhibit. An engineer is troubleshooting an MPLS LDP session between two routers. The output shows that the LDP session is operational. However, MPLS labels are not being exchanged. What is the most likely cause?
hard
A.The peer's addresses are not bound to the LDP Ident.
B.The TCP connection is not using port 646.
✓ C.The LDP label advertisement mode is 'downstream on demand'.
D.The LDP session is not established.
Why C: The exhibit shows that the LDP session is operational (state = Operational), but no labels are being exchanged. In 'downstream on demand' mode, a router does not advertise labels unless explicitly requested by an upstream neighbor via a Label Request message. Since the session is up but no labels are exchanged, this mismatch in label advertisement mode is the most likely cause.
Last reviewed: Jun 11, 2026
Question Discussion
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