- A
The LDP router-id is not reachable via the IGP.
Correct because LDP uses TCP to establish the session, and the router-id must be reachable; if the IGP does not have a route to the peer's LDP router-id, the TCP connection fails.
- B
The mpls label protocol ldp command is missing globally.
Why wrong: Incorrect because LDP hellos are being sent, indicating LDP is enabled globally; the issue is with session establishment.
- C
The interface is configured with mpls ldp igp sync.
Why wrong: Incorrect because LDP-IGP synchronization affects label advertisement and IGP metric, not LDP session formation.
- D
The LDP session is using a non-default transport address.
Why wrong: Incorrect because if the transport address is non-default, it must be reachable; the issue is reachability, not the address itself.
Quick Answer
The answer is that the LDP router-id is not reachable via the IGP. This is correct because while LDP hellos are multicast and succeed over a directly connected link, the actual LDP session requires a TCP connection on port 646 to the transport address—typically the LDP router-id. If that router-id is not in the IGP routing table, the TCP handshake fails, leaving the session stuck in the discovery phase. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of the separation between LDP discovery and session establishment; a common trap is assuming that receiving hellos guarantees a session will form. Remember that hellos use UDP multicast, but the session uses TCP unicast to the router-id. A quick memory tip: "Hellos are local, sessions need a route." Always verify reachability to the LDP router-id with show ip route before diving into ACL or MTU checks.
300-410 MPLS Operations Practice Question
This 300-410 practice question tests your understanding of mpls operations. 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.
A network engineer is troubleshooting MPLS LDP where the LDP session between two directly connected routers is not forming. The engineer runs show mpls ldp discovery and sees that LDP hellos are being sent and received on the link. However, show mpls ldp neighbor shows no neighbors. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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
The LDP router-id is not reachable via the IGP.
LDP hellos are exchanged, but the session does not form, indicating a problem with the transport connection. The most common cause is that the routers cannot establish a TCP connection to the LDP transport address, often due to a missing route to the LDP router-id or an ACL blocking TCP port 646.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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 LDP router-id is not reachable via the IGP.
Why this is correct
Correct because LDP uses TCP to establish the session, and the router-id must be reachable; if the IGP does not have a route to the peer's LDP router-id, the TCP connection fails.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
The mpls label protocol ldp command is missing globally.
Why it's wrong here
Incorrect because LDP hellos are being sent, indicating LDP is enabled globally; the issue is with session establishment.
- ✗
The interface is configured with mpls ldp igp sync.
Why it's wrong here
Incorrect because LDP-IGP synchronization affects label advertisement and IGP metric, not LDP session formation.
- ✗
The LDP session is using a non-default transport address.
Why it's wrong here
Incorrect because if the transport address is non-default, it must be reachable; the issue is reachability, not the address itself.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
- OSPF neighbours must agree on key parameters.
- Router ID selection can affect neighbour relationships and LSDB output.
- OSPF cost influences the preferred path.
- A route can appear in OSPF information but not become the installed route.
TExam Day Tips
- Check area mismatch first when OSPF adjacency fails.
- Review passive interfaces when a network is advertised but no neighbour forms.
- Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
What to study next
Got this wrong? Here's your next step.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this 300-410 question test?
MPLS Operations — This question tests MPLS Operations — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The LDP router-id is not reachable via the IGP. — LDP hellos are exchanged, but the session does not form, indicating a problem with the transport connection. The most common cause is that the routers cannot establish a TCP connection to the LDP transport address, often due to a missing route to the LDP router-id or an ACL blocking TCP port 646.
What should I do if I get this 300-410 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 18, 2026
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