The answer is that R1’s interface is missing the mpls ip command, which directly prevents LDP neighbor discovery. This command is essential because it enables both MPLS forwarding and the transmission of LDP hello messages over UDP port 646; without it, R1 never advertises itself to R2, so the LDP session cannot form and show mpls ldp neighbor returns nothing. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that LDP relies on interface-level MPLS enablement, not just global configuration—a common trap is assuming a global mpls ip command suffices. The key distinction is that each participating interface requires the explicit mpls ip command to send hellos and establish neighbors. Memory tip: think “no mpls ip, no hello, no neighbor”—the interface must speak MPLS to be heard.
300-410 IPv4 Access Control Lists Practice Question
This 300-410 practice question tests your understanding of ipv4 access control lists. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
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?
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.
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.
✗
R1 and R2 are in different MPLS domains.
Why it's wrong here
MPLS domains are not a concept; LDP works per interface.
✓
R1's interface lacks the mpls ip command, preventing LDP hello transmission.
Why this is correct
mpls ip enables LDP on the interface; missing it stops LDP.
Related concept
Read the scenario before looking for a memorised answer.
✗
LDP router IDs are not reachable.
Why it's wrong here
LDP uses transport address; if not reachable, neighbor may not form, but the primary issue is missing mpls ip.
✗
The label distribution protocol is set to TDP instead of LDP.
Why it's wrong here
TDP is Cisco proprietary; LDP is standard, but both require mpls ip.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Detailed technical explanation
How to think about this question
LDP uses UDP hello messages (destination port 646, multicast address 224.0.0.2) to discover neighbors on directly connected links. The 'mpls ip' command enables both MPLS forwarding and the generation of these hellos on that interface. Without it, the interface remains in pure IP forwarding mode, and no LDP adjacency can form. In real-world scenarios, this is a common misconfiguration when enabling MPLS on a new link, often overlooked during initial setup.
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.
TExam Day Tips
→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 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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
IPv4 Access Control Lists — This question tests IPv4 Access Control Lists — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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