- 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.
LDP Label Distribution Fails When mpls ip is Missing on an Interface
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?
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.
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 root cause is that the interface between R2 and R3 is missing the 'mpls ip' command. Without this command on the link, LDP cannot form a session or exchange label bindings across that interface. Since R2 and R3 are not LDP neighbors, R2 never receives labels for prefixes from R3, even though R1 has learned those bindings via its own LDP session with R2.
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.
- ✓
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
Read the scenario before looking for a memorised answer.
- ✗
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: answer the scenario, not the keyword
Cisco often tests the distinction between LDP neighbor state (OPERATIONAL) and the presence of label bindings in the forwarding table, tricking candidates into thinking that a working LDP session on one link guarantees label exchange on all links.
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
LDP uses UDP for neighbor discovery (hello messages on link-local multicast 224.0.0.2) and TCP for session establishment (port 646). The 'mpls ip' command enables both LDP hello processing and label binding exchange on that interface. Without it, the interface is MPLS-unaware, so LDP hellos are ignored and no TCP session can form, even if other interfaces have LDP enabled. In real-world scenarios, this is a common misconfiguration when adding a new link to an existing MPLS network.
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.
What to study next
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FAQ
Questions learners often ask
What does this 300-410 question test?
Device Management — This question tests Device Management — Read the scenario before looking for a memorised answer..
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 root cause is that the interface between R2 and R3 is missing the 'mpls ip' command. Without this command on the link, LDP cannot form a session or exchange label bindings across that interface. Since R2 and R3 are not LDP neighbors, R2 never receives labels for prefixes from R3, even though R1 has learned those bindings via its own LDP session with R2.
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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Last reviewed: Jul 4, 2026
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