hardMultiple ChoiceObjective-mapped
300-410 Practice Question: An MPLS network with EIGRP as the IGP is…
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?
⚠ Common exam trap
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.
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.
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.
- ✗
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.
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.
Go deeper
Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
Key term
MPLS Label Distribution
MPLS Label Distribution is the process by which routers exchange labels that tell them how to forward packets across a network without looking at the IP address each time.
Key term
LDP Protocol
LDP, or Label Distribution Protocol, is a protocol that routers use to automatically exchange labels that enable MPLS (Multiprotocol Label Switching) to create fast, efficient paths for data packets across a network.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.