- A
R1's LDP router-id is not configured, so it uses the IP of the interface facing R2, which may not be reachable from R2's loopback.
LDP requires a reachable router-id. Without explicit configuration, R1 uses the interface IP, which may not be in R2's routing table.
- B
R2 has a loopback interface that is not advertised via EIGRP, so R1 cannot reach it.
Why wrong: The issue is LDP session establishment, not reachability to the loopback.
- C
R1 has an ACL that blocks LDP UDP packets (port 646).
Why wrong: No ACL is mentioned.
- D
R2's 'force' keyword causes it to use the loopback even if it is not reachable.
Why wrong: The 'force' keyword ensures the router-id is used, but it does not cause failure.
Question 2,065 of 2,152
LDP Neighbor Failure: Missing Router-ID
This 300-410 practice question tests your understanding of eigrp troubleshooting. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 EIGRP as the IGP is experiencing label distribution failures. Router R1 shows: 'show mpls ldp neighbor' does not list R2. R1's configuration: mpls ip on interfaces, but no router-id configured. R2's configuration: mpls ldp router-id Loopback0 force. R1 and R2 are directly connected. What is the root cause?
Quick Answer
The answer is that R1’s LDP neighbor is not forming because it lacks an explicitly configured LDP router-id, causing it to default to the IP address of the interface facing R2, which is not reachable from R2’s loopback-based router-id. In MPLS LDP, the router-id must be a stable, reachable IP address—typically a loopback—so that both peers can establish a TCP session on port 646. Without a configured router-id on R1, LDP picks the highest interface IP, which in this case is the directly connected link address; R2, with its forced loopback router-id, cannot reach that interface IP from its own loopback, breaking the neighbor relationship. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that LDP session formation depends on IP reachability between router-ids, not just physical adjacency—a common trap is assuming directly connected interfaces are sufficient. Remember the memory tip: “LDP needs a loopback to talk back; without one, the neighbor is gone.”
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
✓
R1's LDP router-id is not configured, so it uses the IP of the interface facing R2, which may not be reachable from R2's loopback.
R1 has no explicit LDP router-id configured, so it defaults to the highest IP address on a loopback interface or, if none exists, the highest IP on a physical interface. Since R1 has no loopback, it uses the IP of the interface facing R2. R2's LDP router-id is forced to its Loopback0 address via the 'force' keyword. For LDP sessions to establish, each router must be able to reach the other's LDP router-id. R2's loopback may not be reachable from R1's interface IP, or R1's interface IP may not be reachable from R2's loopback, breaking the TCP transport required for LDP.
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's LDP router-id is not configured, so it uses the IP of the interface facing R2, which may not be reachable from R2's loopback.
Why this is correct
LDP requires a reachable router-id. Without explicit configuration, R1 uses the interface IP, which may not be in R2's routing table.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
R2 has a loopback interface that is not advertised via EIGRP, so R1 cannot reach it.
Why it's wrong here
The issue is LDP session establishment, not reachability to the loopback.
- ✗
R1 has an ACL that blocks LDP UDP packets (port 646).
Why it's wrong here
No ACL is mentioned.
- ✗
R2's 'force' keyword causes it to use the loopback even if it is not reachable.
Why it's wrong here
The 'force' keyword ensures the router-id is used, but it does not cause failure.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the nuance that LDP router-id reachability is required for session establishment, and candidates mistakenly focus on the 'force' keyword or ACLs instead of the fundamental TCP reachability requirement.
Trap categories for this question
Keyword trap
The 'force' keyword ensures the router-id is used, but it does not cause failure.
Detailed technical explanation
How to think about this question
LDP uses TCP port 646 for session establishment after initial UDP hello discovery. The LDP router-id must be an IP address that is reachable from the peer; if the router-id is not reachable, the TCP connection fails. When no explicit router-id is configured, Cisco IOS selects the highest loopback IP, or if none, the highest physical interface IP. The 'force' keyword on R2 overrides the default selection but does not guarantee reachability. In real-world scenarios, this often occurs when loopback interfaces are not included in the IGP or when interface IPs are used as router-ids and are not routable across the 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.
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
- →
EIGRP Troubleshooting — study guide chapter
Learn the concepts, then practise the questions
- →
EIGRP Troubleshooting practice questions
Targeted practice on this topic area only
- →
All 300-410 questions
2,152 questions across all exam domains
- →
Cisco CCNP ENARSI 300-410 study guide
Full concept coverage aligned to exam objectives
- →
300-410 practice test guide
How to use practice tests most effectively before exam day
Related practice questions
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.
FAQ
Questions learners often ask
What does this 300-410 question test?
EIGRP Troubleshooting — This question tests EIGRP Troubleshooting — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: R1's LDP router-id is not configured, so it uses the IP of the interface facing R2, which may not be reachable from R2's loopback. — R1 has no explicit LDP router-id configured, so it defaults to the highest IP address on a loopback interface or, if none exists, the highest IP on a physical interface. Since R1 has no loopback, it uses the IP of the interface facing R2. R2's LDP router-id is forced to its Loopback0 address via the 'force' keyword. For LDP sessions to establish, each router must be able to reach the other's LDP router-id. R2's loopback may not be reachable from R1's interface IP, or R1's interface IP may not be reachable from R2's loopback, breaking the TCP transport required for LDP.
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.
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 →
Keep practising
More 300-410 practice questions
- Drag and drop the steps to negotiate an IKEv2 IPsec site-to-site tunnel into the correct order, from first to last.
- Drag and drop the steps to troubleshoot an IPsec site-to-site VPN adjacency failure into the correct order, from first t…
- Drag and drop the steps to verify and validate the operational state of an IPsec site-to-site VPN into the correct order…
- Consider the following configuration snippet: ip cef ! interface GigabitEthernet0/0 ip address 10.0.0.1 255.255.255.25…
- A router is configured with 'logging host 10.1.1.100' and 'logging trap informational'. The engineer notices that syslog…
- Drag and drop the steps to configure a GRE tunnel for IPv6 over IPv4 into the correct order, from first to last.
Last reviewed: Jul 4, 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.
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.
Sign in to join the discussion.