- A
The interface between R2 and R3 is missing the 'mpls ip' command, preventing LDP from establishing a session and exchanging labels.
LDP must be enabled on each interface where label exchange is desired; without it, no session forms, and no labels are exchanged.
- B
The LDP router ID on R2 is set to a loopback that is not reachable from R3.
Why wrong: If the router ID were unreachable, the LDP session would not establish, but the scenario shows R2 and R3 are connected; the issue is likely the interface configuration.
- C
R3 is using a different label distribution protocol, such as TDP.
Why wrong: TDP is deprecated; modern networks use LDP, and the scenario assumes LDP.
- D
The MPLS label range on R2 is exhausted, preventing new label bindings.
Why wrong: Label exhaustion would affect all prefixes, not just those from R3.
Quick Answer
The answer is that the interface between R2 and R3 is missing the **mpls ip** command, which prevents LDP from establishing a session and exchanging labels. In MPLS, the `mpls ip` interface-level command is required to enable LDP label distribution on a specific link; without it, LDP will not form a neighbor relationship or exchange label bindings across that interface. This scenario tests your understanding of LDP label distribution failure in the Cisco CCNP ENARSI 300-410 exam, where a common trap is assuming that a global `mpls ip` configuration is sufficient—it must be applied per interface. Since R1 sees bindings for R3’s prefixes, R3 is advertising them, but R2 cannot receive them because the LDP session on the R2–R3 link is down. A useful memory tip: **“No mpls ip, no LDP flip”**—if the command is missing on the interface, labels won’t be exchanged.
300-410 Device Management Practice Question
This 300-410 practice question tests your understanding of device management. 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.
An MPLS network with routers R1, R2, and R3 is experiencing label distribution failures. R1 and R2 are LDP neighbors, but R2 shows: 'show mpls ldp neighbor' shows R1 in state OPERATIONAL, but 'show mpls forwarding-table' shows no labels for prefixes from R3. R3 is connected to R2 via a different interface. R2 configuration: mpls ip on both interfaces. R1 shows: 'show mpls ldp bindings' includes prefixes from R3. What is the root cause?
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 interface between R2 and R3 is missing the 'mpls ip' command, preventing LDP from establishing a session and exchanging labels.
The issue is that LDP is working between R1 and R2, but R2 is not receiving labels for prefixes from R3. This could be due to LDP not being enabled on the link between R2 and R3, or a mismatch in LDP router IDs. The fact that R1 has bindings for R3's prefixes suggests that R3 is advertising them to R1, but R2 is not receiving them. The root cause is likely that the LDP session between R2 and R3 is not established due to a missing 'mpls ip' on the interface or an LDP router ID issue.
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 interface between R2 and R3 is missing the 'mpls ip' command, preventing LDP from establishing a session and exchanging labels.
Why this is correct
LDP must be enabled on each interface where label exchange is desired; without it, no session forms, and no labels are exchanged.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
The LDP router ID on R2 is set to a loopback that is not reachable from R3.
Why it's wrong here
If the router ID were unreachable, the LDP session would not establish, but the scenario shows R2 and R3 are connected; the issue is likely the interface configuration.
- ✗
R3 is using a different label distribution protocol, such as TDP.
Why it's wrong here
TDP is deprecated; modern networks use LDP, and the scenario assumes LDP.
- ✗
The MPLS label range on R2 is exhausted, preventing new label bindings.
Why it's wrong here
Label exhaustion would affect all prefixes, not just those from R3.
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.
Trap categories for this question
Command / output trap
If the router ID were unreachable, the LDP session would not establish, but the scenario shows R2 and R3 are connected; the issue is likely the interface configuration.
Scenario analysis trap
If the router ID were unreachable, the LDP session would not establish, but the scenario shows R2 and R3 are connected; the issue is likely the interface configuration.
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?
Device Management — This question tests Device Management — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The interface between R2 and R3 is missing the 'mpls ip' command, preventing LDP from establishing a session and exchanging labels. — The issue is that LDP is working between R1 and R2, but R2 is not receiving labels for prefixes from R3. This could be due to LDP not being enabled on the link between R2 and R3, or a mismatch in LDP router IDs. The fact that R1 has bindings for R3's prefixes suggests that R3 is advertising them to R1, but R2 is not receiving them. The root cause is likely that the LDP session between R2 and R3 is not established due to a missing 'mpls ip' on the interface or an LDP router ID issue.
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.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 18, 2026
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.
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