- A
LDP uses UDP to send Hello messages for neighbor discovery.
Correct because LDP Hellos are sent as UDP packets to the multicast address 224.0.0.2 on port 646.
- B
LDP establishes TCP sessions between LSRs to exchange label bindings.
Correct because after discovery, LDP uses TCP (port 646) for reliable label exchange.
- C
LDP assigns labels to all prefixes in the routing table by default.
Correct because LDP by default allocates labels for every route in the IP routing table.
- D
LDP supports traffic engineering by reserving bandwidth along LSPs.
Why wrong: Incorrect because LDP does not support traffic engineering; RSVP-TE is used for that.
- E
LDP uses OSPF to distribute label bindings across the network.
Why wrong: Incorrect because LDP uses its own protocol (UDP/TCP) for label distribution, not OSPF.
CCNP MPLS Practice Question
This 350-401 practice question tests your understanding of mpls. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
Which three statements about LDP (Label Distribution Protocol) are true? (Choose three.)
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 uses UDP to send Hello messages for neighbor discovery.
LDP uses Hello messages (UDP on port 646) for neighbor discovery and TCP (port 646) for session establishment and label exchange. LDP assigns labels to all prefixes in the routing table by default. LDP does not support traffic engineering; RSVP-TE is used for that. LDP does not use OSPF for label distribution; it has its own discovery and session mechanisms.
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 uses UDP to send Hello messages for neighbor discovery.
- ✓
LDP establishes TCP sessions between LSRs to exchange label bindings.
- ✓
LDP assigns labels to all prefixes in the routing table by default.
Why this is correct
Correct because LDP by default allocates labels for every route in the IP routing table.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
LDP supports traffic engineering by reserving bandwidth along LSPs.
Why it's wrong here
Incorrect because LDP does not support traffic engineering; RSVP-TE is used for that.
- ✗
LDP uses OSPF to distribute label bindings across the network.
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 350-401 OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this 350-401 question test?
MPLS — This question tests MPLS — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: LDP uses UDP to send Hello messages for neighbor discovery. — LDP uses Hello messages (UDP on port 646) for neighbor discovery and TCP (port 646) for session establishment and label exchange. LDP assigns labels to all prefixes in the routing table by default. LDP does not support traffic engineering; RSVP-TE is used for that. LDP does not use OSPF for label distribution; it has its own discovery and session mechanisms.
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.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 18, 2026
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