350-501 MPLS and Segment Routing Practice Question
An engineer is configuring MPLS LDP on a Cisco router. Which TWO statements about LDP operation are correct? (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 discovers neighbors using UDP hello messages on port 646, and then establishes a TCP session on port 646.
LDP uses downstream unsolicited mode by default in Cisco IOS; liberal label retention means it keeps all labels even if not directly reachable. Option B is correct because LDP uses UDP hello on port 646 for neighbor discovery and then TCP on port 646 for session establishment. Option D is correct because liberal label retention stores all received labels, even from non-next-hop neighbors, improving convergence. Option C is incorrect because LDP uses downstream unsolicited, not downstream-on-demand. Options A and E are incorrect.
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 LFIB (Label Forwarding Information Base) in LDP is used to store only the outgoing label mapping for each FEC.
Why it's wrong here
Incorrect. LFIB stores incoming label, outgoing label, and interface.
- ✓
LDP discovers neighbors using UDP hello messages on port 646, and then establishes a TCP session on port 646.
Why this is correct
Correct. LDP uses UDP for discovery, TCP for session.
- ✗
LDP uses a downstream-on-demand label distribution method, where labels are sent only when requested by an upstream router.
Why it's wrong here
Incorrect. Cisco uses downstream unsolicited by default.
- ✓
In liberal label retention mode, LDP stores all received labels, even from non-next-hop neighbors, allowing faster convergence.
Why this is correct
Correct. Liberal retention stores all labels.
- ✗
LDP label allocation happens only after the IGP converges, and labels are allocated for all IGP routes by default.
Why it's wrong here
Incorrect. LDP allocates labels for all routes in the routing table, not only IGP.
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Written by Johnson Ajibi, MSc IT Security
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