- A
MPLS forwarding uses the Label Forwarding Information Base (LFIB) to make forwarding decisions.
Correct because the LFIB is the table used for MPLS label switching.
- B
The LFIB is populated by label distribution protocols such as LDP.
Correct because LDP and other protocols distribute labels and populate the LFIB.
- C
The LIB stores all labels learned from LDP neighbors, but the LFIB is used for actual forwarding.
Correct because the LIB contains all label bindings, while the LFIB contains only the best entries for forwarding.
- D
MPLS forwarding uses the Forwarding Information Base (FIB) for label lookups.
Why wrong: Incorrect because the FIB is used for IP forwarding, not MPLS label switching.
- E
MPLS forwarding uses the Routing Information Base (RIB) to determine the next hop.
Why wrong: Incorrect because the RIB is for IP routing; MPLS forwarding uses the LFIB.
Quick Answer
The answer is that the LFIB is used for actual MPLS forwarding, while the LIB stores all labels learned from LDP neighbors. This is correct because MPLS routers make forwarding decisions by performing a label lookup in the LFIB, which is populated by label distribution protocols like LDP or RSVP-TE; the LIB merely holds all received labels without dictating forwarding actions. On the ENCOR 350-401 exam, this concept tests your ability to distinguish between the control plane (LIB) and data plane (LFIB), and a common trap is confusing the FIB—which handles IP forwarding—with MPLS label switching. Remember that the RIB and FIB are for IP routing, not MPLS. A helpful memory tip: think of the LIB as a library of all known labels, but the LFIB is the active map you actually follow to forward packets.
350-401 MPLS Practice Question
This 350-401 practice question tests your understanding of mpls. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
Which three statements about MPLS forwarding are true? (Choose three.)
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
MPLS forwarding uses the Label Forwarding Information Base (LFIB) to make forwarding decisions.
MPLS forwarding is based on a label lookup in the LFIB (Label Forwarding Information Base). The LFIB is populated by label distribution protocols such as LDP or RSVP-TE. The FIB (Forwarding Information Base) is used for IP forwarding, not MPLS. The LIB (Label Information Base) stores all labels learned from neighbors, but the LFIB is used for actual forwarding decisions. Option D is incorrect because the FIB is not used for MPLS forwarding. Option E is incorrect because the RIB (Routing Information Base) is used for IP routing, not MPLS label switching.
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.
- ✓
MPLS forwarding uses the Label Forwarding Information Base (LFIB) to make forwarding decisions.
Why this is correct
Correct because the LFIB is the table used for MPLS label switching.
Related concept
OSPF neighbours must agree on key parameters.
- ✓
The LFIB is populated by label distribution protocols such as LDP.
Why this is correct
Correct because LDP and other protocols distribute labels and populate the LFIB.
Related concept
OSPF neighbours must agree on key parameters.
- ✓
The LIB stores all labels learned from LDP neighbors, but the LFIB is used for actual forwarding.
Why this is correct
Correct because the LIB contains all label bindings, while the LFIB contains only the best entries for forwarding.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
MPLS forwarding uses the Forwarding Information Base (FIB) for label lookups.
Why it's wrong here
Incorrect because the FIB is used for IP forwarding, not MPLS label switching.
- ✗
MPLS forwarding uses the Routing Information Base (RIB) to determine the next hop.
Why it's wrong here
Incorrect because the RIB is for IP routing; MPLS forwarding uses the LFIB.
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.
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 350-401 OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this 350-401 question test?
MPLS — This question tests MPLS — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: MPLS forwarding uses the Label Forwarding Information Base (LFIB) to make forwarding decisions. — MPLS forwarding is based on a label lookup in the LFIB (Label Forwarding Information Base). The LFIB is populated by label distribution protocols such as LDP or RSVP-TE. The FIB (Forwarding Information Base) is used for IP forwarding, not MPLS. The LIB (Label Information Base) stores all labels learned from neighbors, but the LFIB is used for actual forwarding decisions. Option D is incorrect because the FIB is not used for MPLS forwarding. Option E is incorrect because the RIB (Routing Information Base) is used for IP routing, not MPLS label switching.
What should I do if I get this 350-401 question wrong?
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 350-401 OSPF questions on adjacency and route selection.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 18, 2026
This 350-401 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 350-401 exam.
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