The answer is that the routes are not present in the global routing table on the router. LDP by default only assigns labels to IGP routes that are actively installed in the routing table; if a prefix learned via OSPF or IS-IS is missing due to summarization, route filtering, or a missing network statement, LDP will not generate a label binding for it. This is a common trap on the ENCOR 350-401 exam, where candidates assume that an established LDP session guarantees label assignments for all IGP routes, but LDP strictly follows the routing table—not the link-state database. The question tests your understanding that LDP’s label allocation is tied to the presence of a usable route in the global RIB, not to the IGP adjacency or BGP. A quick memory tip: “No route, no label—LDP follows the RIB, not the IGP.”
350-401 MPLS Practice Question
This 350-401 practice question tests your understanding of mpls. 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.
A service provider is migrating from a traditional IP core to an MPLS core. The engineer has configured LDP on all core routers and verified that LDP sessions are established. However, some prefixes learned via OSPF are not being assigned labels. The 'show mpls ldp bindings' command shows missing bindings for certain routes. 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
✓
The routes are not present in the global routing table on the router.
LDP by default only assigns labels to routes in the routing table that are not BGP routes. If the routes are not in the routing table (e.g., due to summarization or filtering), LDP will not assign labels. Option A is correct. Option B is wrong because LDP does not require IGP; Option C is wrong because LDP can assign labels to any route; Option D is wrong because LDP does not require BGP.
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 routes are not present in the global routing table on the router.
Why this is correct
Correct because LDP only assigns labels to routes that are in the routing table.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Incorrect because LDP does not require redistribution; it uses the routing table directly.
✗
LDP is configured to only assign labels to BGP routes.
Why it's wrong here
Incorrect because LDP typically assigns labels to IGP routes as well.
✗
The 'mpls ldp autoconfig' command is missing on OSPF.
Why it's wrong here
Incorrect because autoconfig is used to enable LDP on interfaces, not to assign labels to routes.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 350-401 question in full detail.
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 350-401 OSPF questions on adjacency and route selection.
MPLS — This question tests MPLS — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The routes are not present in the global routing table on the router. — LDP by default only assigns labels to routes in the routing table that are not BGP routes. If the routes are not in the routing table (e.g., due to summarization or filtering), LDP will not assign labels. Option A is correct. Option B is wrong because LDP does not require IGP; Option C is wrong because LDP can assign labels to any route; Option D is wrong because LDP does not require BGP.
What should I do if I get this 350-401 question wrong?
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 350-401 OSPF questions on adjacency and route selection.
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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