Question 1,706 of 2,152

MPLS Operations →mediumMultiple ChoiceObjective-mapped

Quick Answer

The answer is that LDP is not enabled on the interface facing the next-hop router. This is correct because in MPLS L3VPN troubleshooting, when a VRF route for CE2’s subnet is present in the routing table but the corresponding MPLS label is missing in the LFIB, it indicates that LDP has not assigned a label for the BGP next-hop of that VPN route. Even though the IGP route to that next-hop exists and LDP neighbors are up, the label binding fails if LDP is not explicitly enabled on the specific interface connecting to the next-hop router. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of the MPLS label allocation process and the dependency between LDP and the IGP next-hop interface. A common trap is assuming that LDP neighbors being up guarantees label bindings for all routes, but LDP must be enabled per interface. Memory tip: “Route present, label absent? Check the interface LDP sent.”

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 L3VPN where CE1 cannot reach CE2. The PE routers are running OSPF as the IGP and LDP for label distribution. On PE1, the engineer sees that the VRF route for CE2's subnet is present, but the corresponding MPLS label is missing in the LFIB. The show mpls ldp neighbor command shows LDP neighbors are up. What is the most likely cause of the missing label?

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

Review the full OSPF breakdown →

A
LDP is not enabled on the interface facing the next-hop router.
Correct because LDP must be enabled on the interface that connects to the next-hop router to assign a label for the IGP route, which is used to resolve the BGP next-hop in MPLS VPN.
B
The VRF route is not redistributed into BGP on the remote PE.
Why wrong: Incorrect because the VRF route is present on PE1, indicating redistribution is working; the issue is label binding, not route advertisement.
C
MTU mismatch on the link between PE1 and P causes label imposition failure.
Why wrong: Incorrect because an MTU mismatch would cause packet drops, not a missing label in the LFIB; LDP would still assign a label.
D
The mpls label protocol ldp command is missing under the VRF.
Why wrong: Incorrect because LDP is configured globally or per interface, not under a VRF; VRF configuration does not affect LDP label assignment.

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

✓

LDP is not enabled on the interface facing the next-hop router.

The missing label in the LFIB despite the VRF route being present indicates that LDP is not binding a label to the BGP next-hop of the VPN route. Since the IGP route to the BGP next-hop is present but LDP has not assigned a label for it, the issue is likely that LDP is not enabled on the interface facing the next-hop router.

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.

✓
LDP is not enabled on the interface facing the next-hop router.
Why this is correct
Correct because LDP must be enabled on the interface that connects to the next-hop router to assign a label for the IGP route, which is used to resolve the BGP next-hop in MPLS VPN.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
✗
The VRF route is not redistributed into BGP on the remote PE.
Why it's wrong here
Incorrect because the VRF route is present on PE1, indicating redistribution is working; the issue is label binding, not route advertisement.
✗
MTU mismatch on the link between PE1 and P causes label imposition failure.
Why it's wrong here
Incorrect because an MTU mismatch would cause packet drops, not a missing label in the LFIB; LDP would still assign a label.
✗
The mpls label protocol ldp command is missing under the VRF.
Why it's wrong here
Incorrect because LDP is configured globally or per interface, not under a VRF; VRF configuration does not affect LDP label assignment.

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: LDP is not enabled on the interface facing the next-hop router. — The missing label in the LFIB despite the VRF route being present indicates that LDP is not binding a label to the BGP next-hop of the VPN route. Since the IGP route to the BGP next-hop is present but LDP has not assigned a label for it, the issue is likely that LDP is not enabled on the interface facing the next-hop router.

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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