hardMultiple Choice
300-410 Practice Question: MPLS network: LDP neighbors are down between R1…
MPLS network: LDP neighbors are down between R1 and R2. R1 shows: show mpls ldp neighbor includes nothing. R2 has: interface GigabitEthernet0/0, mpls ip, but R1 has no mpls ip on its interface. What is the root cause?
⚠ Common exam trap
Cisco often tests the specific requirement of the 'mpls ip' interface command for LDP neighbor discovery, leading candidates to incorrectly focus on reachability or protocol version issues when the problem is a missing interface-level command.
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
✓
R1's interface lacks the mpls ip command, preventing LDP hello transmission.
The root cause is that R1's interface is missing the 'mpls ip' command. This command is required on each interface to enable MPLS forwarding and to send LDP hello messages (UDP port 646) to neighbors. Without it, R1 cannot discover R2 as an LDP neighbor, so the LDP session never forms, and 'show mpls ldp neighbor' returns nothing on R1.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
R1 and R2 are in different MPLS domains.
Why it's wrong here
Both routers sit in the same routing domain; the actual fault is that R1's interface lacks the mpls ip command, so no LDP hellos are sent or received. Different MPLS domains is tempting because domain mismatches do break LDP sessions, but that would require separate autonomous systems or VRFs.
- ✓
R1's interface lacks the mpls ip command, preventing LDP hello transmission.
Why this is correct
LDP discovers neighbours by exchanging hello messages on interfaces where MPLS is enabled. Without the mpls ip command on R1's interface, no hellos are sent or received, so no neighbour session forms and show mpls ldp neighbor returns nothing — exactly the stem's symptom.
- ✗
LDP router IDs are not reachable.
Why it's wrong here
Unreachable LDP router IDs would prevent session establishment only after interfaces run LDP; here R1 has no mpls ip, so no transport address is advertised. It tempts because router ID reachability is a genuine LDP requirement, typically resolved via a loopback advertised into the IGP.
- ✗
The label distribution protocol is set to TDP instead of LDP.
Why it's wrong here
TDP and LDP are both label distribution protocols and either can form sessions; the stem shows R1's interface simply has no mpls ip configured, so no protocol runs at all. TDP mismatch tempts because protocol incompatibility genuinely prevents adjacency, but it requires both interfaces to be MPLS-enabled first.
Go deeper
Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
Key term
MPLS Label Distribution
MPLS Label Distribution is the process by which routers exchange labels that tell them how to forward packets across a network without looking at the IP address each time.
Key term
LDP Protocol
LDP, or Label Distribution Protocol, is a protocol that routers use to automatically exchange labels that enable MPLS (Multiprotocol Label Switching) to create fast, efficient paths for data packets across a network.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 300-410 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 300-410 exam.