mediumMultiple SelectObjective-mapped
350-401 Practice Question: Which two statements about MPLS label…
Which two statements about MPLS label distribution protocol (LDP) are true? (Choose two.)
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 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.
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.
- ✓
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.
- ✗
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.
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Same concept, more angles
1 more way this is tested on 350-401
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.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-401 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-401 exam.