hardMultiple ChoiceObjective-mapped
300-410 Practice Question: An MPLS network is experiencing label…
An MPLS network is experiencing label distribution failures. Router R1 is an LSR connected to R2. R1's show mpls ldp neighbor shows R2 in OPERATIONAL state, but show mpls ldp bindings shows no label bindings for prefixes learned via OSPF from R2. R1's mpls ldp router-id is 1.1.1.1, and R2's is 2.2.2.2. The OSPF process on R1 advertises the loopback0 interface with ip address 1.1.1.1 255.255.255.255, and R2's loopback0 is 2.2.2.2. The link between them is 192.168.1.0/30. What is the root cause?
⚠ Common exam trap
Cisco often tests the distinction between LDP neighbor adjacency (which can form using link-local addresses) and the actual exchange of label bindings, which requires the LDP router-id to be reachable via the IGP; candidates mistakenly assume an OPERATIONAL neighbor state guarantees full label exchange.
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 LDP router-id on R1 is not reachable from R2 because OSPF is not advertising the loopback0 route, causing LDP to not exchange label bindings.
R1 shows R2 as an LDP neighbor in OPERATIONAL state, meaning the LDP session (TCP port 646) is established. However, no label bindings are exchanged for OSPF-learned prefixes from R2. LDP uses the router-id (1.1.1.1) as the transport address for label binding exchange. If OSPF does not advertise R1's loopback0 (1.1.1.1/32), R2 cannot reach this address, so LDP cannot complete the label mapping exchange, even though the neighbor session is up. The correct root cause is that R1's LDP router-id is not reachable from R2.
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 LDP router-id on R1 is not reachable from R2 because OSPF is not advertising the loopback0 route, causing LDP to not exchange label bindings.
Why this is correct
If the router-id is not reachable, LDP session may form using the link address, but label bindings for prefixes learned via OSPF may fail because the transport address is not routable.
- ✗
The mpls ldp label allocation is configured as 'per-prefix' instead of 'per-interface'.
Why it's wrong here
Label allocation mode does not prevent bindings from being exchanged; it affects how labels are assigned.
- ✗
The OSPF process on R1 has a route-map filtering the loopback route.
Why it's wrong here
If the loopback is filtered, the router-id would not be reachable, causing LDP session failure, but the session is OPERATIONAL.
- ✗
The LDP session is using the interface IP address as transport, but OSPF is not advertising the interface network.
Why it's wrong here
The interface network is directly connected, so it is always reachable; the issue is with the router-id.
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
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.