Question 829 of 2,152

Device Management →hardMultiple ChoiceObjective-mapped

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?

Question 1hardmultiple choice

Read the full MPLS explanation →

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.

Full breakdown with real-world context →

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.

Related 300-410 practice-question pages

Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.

Layer 3 Technologies practice questions

Practise 300-410 questions linked to Layer 3 Technologies.

EIGRP Troubleshooting practice questions

Practise 300-410 questions linked to EIGRP Troubleshooting.

OSPF Troubleshooting (v2/v3) practice questions

Practise 300-410 questions linked to OSPF Troubleshooting (v2/v3).

BGP Troubleshooting practice questions

Practise 300-410 questions linked to BGP Troubleshooting.

Route Redistribution practice questions

Practise 300-410 questions linked to Route Redistribution.

Policy-Based Routing (PBR) practice questions

Practise 300-410 questions linked to Policy-Based Routing (PBR).

VRF-Lite practice questions

Practise 300-410 questions linked to VRF-Lite.

Route Maps and Route Filtering practice questions

Practise 300-410 questions linked to Route Maps and Route Filtering.

Administrative Distance practice questions

Practise 300-410 questions linked to Administrative Distance.

Route Summarization practice questions

Practise 300-410 questions linked to Route Summarization.

Bidirectional Forwarding Detection (BFD) practice questions

Practise 300-410 questions linked to Bidirectional Forwarding Detection (BFD).

VPN Technologies practice questions

Practise 300-410 questions linked to VPN Technologies.

Practice this exam

Start a free 300-410 practice session

Short sessions build daily habit. Longer sessions build exam-day stamina. Try a timed session to simulate real conditions.

10 questions 20 questions 30 questions 50 questions Timed 30

300-410 practice-test guide →Study guide →Browse all practice tests

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?

What is the key concept behind this question?

OSPF neighbours must agree on key parameters.

About these practice questions

Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →

How Courseiva writes practice questions · Editorial policy

Keep practising

Question Discussion

Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.

Loading comments…

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.