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300-410 Practice Question: In an MPLS LDP network, routers R1 and R2 are…
In an MPLS LDP network, routers R1 and R2 are directly connected via GigabitEthernet0/0. R1 shows 'show mpls ldp neighbor' output: 'Peer LDP Ident: 10.0.0.2:0, Local LDP Ident: 10.0.0.1:0, TCP connection: 10.0.0.2.646 - 10.0.0.1.646, State: OPERATIONAL' but 'show mpls forwarding-table' for prefix 192.168.1.0/24 shows 'No label'. R2 has the same prefix in its routing table. 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
✓
R2 does not have MPLS enabled on the interface facing the prefix's origin; LDP only labels routes learned via MPLS-enabled interfaces.
LDP neighbors are operational, but label binding may fail if the IGP (e.g., OSPF) has a route to the prefix but LDP does not advertise a label for it. This can occur if the prefix is a connected route on R2 but R2's LDP is not configured to advertise labels for connected routes (e.g., 'no mpls ldp advertise-labels' or route filtering). Alternatively, R2 may have the prefix via a different interface not running MPLS.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
R2 does not have MPLS enabled on the interface facing the prefix's origin; LDP only labels routes learned via MPLS-enabled interfaces.
Why this is correct
If the prefix is learned via an interface without 'mpls ip', LDP will not assign a label, causing R1 to have no label in forwarding table.
- ✗
R1 has an ACL blocking LDP label advertisement for prefix 192.168.1.0/24.
Why it's wrong here
No evidence of ACL; LDP neighbor state is operational.
- ✗
The MTU mismatch between R1 and R2 causes label distribution failure.
Why it's wrong here
MTU affects packet forwarding, not label binding.
- ✗
R1's MPLS forwarding table is full; no room for new labels.
Why it's wrong here
No indication of resource exhaustion.
Visual reference
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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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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