Question 94 of 2,152

MPLS Operations →hardMultiple ChoiceObjective-mapped

300-410 MPLS Operations Practice Question

This 300-410 practice question tests your understanding of mpls operations. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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.

An engineer configures MPLS forwarding on a router. The MPLS LDP neighbors are established, but some prefixes are not being assigned labels. The engineer checks the 'show mpls ldp bindings' and sees that the prefixes are missing. What is the most likely explanation?

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 1hardmultiple choice

Read the full MPLS explanation →

A
The prefix is learned via BGP, and LDP does not assign labels to BGP routes by default.
LDP by default only assigns labels to IGP routes (OSPF, EIGRP, IS-IS). BGP routes are not labeled unless 'mpls ldp autoconfig bgp' or 'label-distribution' is configured for BGP.
B
The 'mpls ldp advertise-labels' command is configured to deny the prefix.
Why wrong: If the prefix is denied by an advertise-labels filter, it would not appear in bindings. However, the question states some prefixes are missing, and this is a possible cause but less likely than the default behavior for BGP routes.
C
The 'mpls ldp label' command is missing on the interface.
Why wrong: The 'mpls ldp label' command is not a valid command; LDP is enabled per interface with 'mpls ip'.
D
The 'tag-switching' is not enabled globally.
Why wrong: MPLS forwarding requires 'mpls ip' globally and per interface. If not enabled, LDP neighbors would not form, but the question states neighbors are established.

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 prefix is learned via BGP, and LDP does not assign labels to BGP routes by default.

MPLS LDP assigns labels to prefixes that are in the routing table and are IGP routes. If a prefix is learned via BGP, LDP will not assign a label to it by default unless the 'mpls ldp autoconfig' or 'label-distribution' is configured for BGP. Additionally, if the prefix is a connected route or a static route, LDP may not assign a label unless explicitly configured. The edge case is that the prefix is a BGP route, and LDP does not label BGP routes by default.

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 prefix is learned via BGP, and LDP does not assign labels to BGP routes by default.
Why this is correct
LDP by default only assigns labels to IGP routes (OSPF, EIGRP, IS-IS). BGP routes are not labeled unless 'mpls ldp autoconfig bgp' or 'label-distribution' is configured for BGP.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
✗
The 'mpls ldp advertise-labels' command is configured to deny the prefix.
Why it's wrong here
If the prefix is denied by an advertise-labels filter, it would not appear in bindings. However, the question states some prefixes are missing, and this is a possible cause but less likely than the default behavior for BGP routes.
✗
The 'mpls ldp label' command is missing on the interface.
Why it's wrong here
The 'mpls ldp label' command is not a valid command; LDP is enabled per interface with 'mpls ip'.
✗
The 'tag-switching' is not enabled globally.
Why it's wrong here
MPLS forwarding requires 'mpls ip' globally and per interface. If not enabled, LDP neighbors would not form, but the question states neighbors are established.

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.

Trap categories for this question

Command / output trap
The 'mpls ldp label' command is not a valid command; LDP is enabled per interface with 'mpls ip'.

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 prefix is learned via BGP, and LDP does not assign labels to BGP routes by default. — MPLS LDP assigns labels to prefixes that are in the routing table and are IGP routes. If a prefix is learned via BGP, LDP will not assign a label to it by default unless the 'mpls ldp autoconfig' or 'label-distribution' is configured for BGP. Additionally, if the prefix is a connected route or a static route, LDP may not assign a label unless explicitly configured. The edge case is that the prefix is a BGP route, and LDP does not label BGP routes by default.

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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