The correct answer is that LDP uses TCP port 646 for session establishment and UDP for hello messages sent to multicast address 224.0.0.2. This is because LDP separates its discovery and session functions: UDP hello messages allow routers to dynamically discover potential LDP peers on directly connected links, while a reliable TCP connection on port 646 is then established to exchange label mappings and maintain the session state. On the ENCOR 350-401 exam, this distinction is a frequent trap—many candidates mistakenly think LDP uses UDP for the entire session or confuse it with RSVP for traffic engineering. A common memory tip is to remember that “hello” is a quick, connectionless discovery (UDP), while the actual label exchange needs reliability (TCP). For quick recall, think “UDP to find, TCP to bind.”
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 two statements about MPLS label distribution protocol (LDP) are true? (Choose two.)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
LDP uses TCP port 646 for session establishment.
LDP uses TCP port 646 for session establishment and UDP for discovery (hello messages). LDP sessions are established between directly connected LSRs by default, but can also be established between non-adjacent LSRs using targeted hellos. LDP assigns labels to every prefix in the routing table by default. Option C is incorrect because LDP uses TCP, not UDP, for session establishment. Option D is incorrect because LDP does not use RSVP; that is for traffic engineering. Option E is incorrect because LDP assigns labels to all prefixes, not just BGP routes.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 TCP port 646 for session establishment.
Why this is correct
Correct because LDP sessions use TCP port 646 for reliable communication.
Related concept
Read the scenario before looking for a memorised answer.
✓
LDP hello messages are sent as UDP packets to multicast address 224.0.0.2.
Why this is correct
Correct because LDP discovery uses UDP hello messages to the all-routers multicast address.
Related concept
Read the scenario before looking for a memorised answer.
✗
LDP uses UDP for session establishment.
Why it's wrong here
Incorrect because LDP uses TCP for session establishment, not UDP.
✗
LDP uses RSVP to distribute labels.
Why it's wrong here
Incorrect because RSVP is used for MPLS-TE, not for basic label distribution by LDP.
✗
LDP assigns labels only to BGP routes.
Why it's wrong here
Incorrect because LDP assigns labels to all IGP routes by default, not only BGP.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the 350-401 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which 350-401 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
MPLS — This question tests MPLS — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: LDP uses TCP port 646 for session establishment. — LDP uses TCP port 646 for session establishment and UDP for discovery (hello messages). LDP sessions are established between directly connected LSRs by default, but can also be established between non-adjacent LSRs using targeted hellos. LDP assigns labels to every prefix in the routing table by default. Option C is incorrect because LDP uses TCP, not UDP, for session establishment. Option D is incorrect because LDP does not use RSVP; that is for traffic engineering. Option E is incorrect because LDP assigns labels to all prefixes, not just BGP routes.
What should I do if I get this 350-401 question wrong?
Identify which 350-401 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Which three statements about LDP (Label Distribution Protocol) are true? (Choose three.)
hard
✓ A.LDP uses UDP to send Hello messages for neighbor discovery.
✓ B.LDP establishes TCP sessions between LSRs to exchange label bindings.
✓ C.LDP assigns labels to all prefixes in the routing table by default.
D.LDP supports traffic engineering by reserving bandwidth along LSPs.
E.LDP uses OSPF to distribute label bindings across the network.
Why A: 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.
Last reviewed: Jun 18, 2026
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