Question 812 of 2,152

MPLS Operations →mediumMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the `mpls ldp neighbor` command is missing on one or both PE routers. This is correct because an MPLS L2VPN pseudowire relies on a targeted LDP session to exchange VC labels between the two endpoints; a standard link LDP session only handles local label bindings and cannot establish the pseudowire. When the targeted LDP session is missing, the pseudowire remains down even though the VC ID matches and the underlying LDP session appears up. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of the distinction between link LDP and targeted LDP, a common trap where candidates assume a single LDP session is sufficient. Remember the memory tip: "Targeted LDP is the bridge for the pseudowire—without the `neighbor` command, the bridge is missing."

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 an MPLS L2VPN (VPWS) where the pseudowire between two PE routers is down. The show mpls l2transport vc command displays state 'down' and the VC ID is correct on both ends. The engineer checks the MPLS LDP session and sees it is up, but the targeted LDP session for the pseudowire is not established. 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.

Question 1mediummultiple choice

Read the full MPLS explanation →

A
The mpls ldp neighbor command is missing on one or both PEs.
Correct because targeted LDP sessions require explicit configuration using the mpls ldp neighbor command to initiate the session for pseudowire signaling.
B
The VC ID does not match on both ends.
Why wrong: Incorrect because the stem states the VC ID is correct on both ends; a mismatch would cause a different symptom.
C
The IGP is not converging, causing reachability issues.
Why wrong: Incorrect because the LDP session is up, indicating IGP reachability is fine; the targeted session is a separate issue.
D
The mpls label protocol ldp command is missing globally.
Why wrong: Incorrect because LDP is already operational (session up), so the global protocol is configured; the targeted session requires additional configuration.

Full breakdown with real-world context →

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 mpls ldp neighbor command is missing on one or both PEs.

For a pseudowire to be established, a targeted LDP session is required between the two PE routers. If the targeted LDP session is not established, the pseudowire cannot exchange labels. The most common cause is that the mpls ldp neighbor command is missing or misconfigured on one or both PEs.

Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.

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 mpls ldp neighbor command is missing on one or both PEs.
Why this is correct
Correct because targeted LDP sessions require explicit configuration using the mpls ldp neighbor command to initiate the session for pseudowire signaling.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
✗
The VC ID does not match on both ends.
Why it's wrong here
Incorrect because the stem states the VC ID is correct on both ends; a mismatch would cause a different symptom.
✗
The IGP is not converging, causing reachability issues.
Why it's wrong here
Incorrect because the LDP session is up, indicating IGP reachability is fine; the targeted session is a separate issue.
✗
The mpls label protocol ldp command is missing globally.
Why it's wrong here
Incorrect because LDP is already operational (session up), so the global protocol is configured; the targeted session requires additional configuration.

Common exam traps

Common exam trap: OSPF can fail even when IP connectivity looks correct

OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.

Detailed technical explanation

How to think about this question

OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.

KKey Concepts to Remember

OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.

TExam Day Tips

Check area mismatch first when OSPF adjacency fails.
Review passive interfaces when a network is advertised but no neighbour forms.
Use show ip ospf neighbor and show ip route clues carefully.

Key takeaway

OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.

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.

Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.

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FAQ

Questions learners often ask

What does this 300-410 question test?

MPLS Operations — This question tests MPLS Operations — OSPF neighbours must agree on key parameters..

What is the correct answer to this question?

The correct answer is: The mpls ldp neighbor command is missing on one or both PEs. — For a pseudowire to be established, a targeted LDP session is required between the two PE routers. If the targeted LDP session is not established, the pseudowire cannot exchange labels. The most common cause is that the mpls ldp neighbor command is missing or misconfigured on one or both PEs.

What should I do if I get this 300-410 question wrong?

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?

OSPF neighbours must agree on key parameters.

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