The answer is LDP, BGP, and RSVP. These three protocols are the standard MPLS label distribution protocols used to advertise and exchange label bindings between routers in an MPLS network, enabling label-switched paths for traffic engineering and VPN services. On the Cisco SPCOR / CCNP Service Provider Core 350-501 exam, this question tests your ability to distinguish between control plane protocols that actually distribute labels and interior gateway protocols like OSPF or IS-IS, which only handle routing and do not directly distribute labels—though they can be used in Segment Routing to advertise prefix-SIDs, that is a different mechanism. A common trap is confusing IGPs with label distribution protocols, so remember that LDP, BGP, and RSVP are the three core label distributors. A helpful memory tip is "LBR" for LDP, BGP, and RSVP—think of it as "Label Binding Routers."
350-501 Architecture Practice Question
This 350-501 practice question tests your understanding of architecture. 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 protocols are used for label distribution in an MPLS network?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
LDP
Options A, B, and C are correct. LDP, BGP, and RSVP are standard label distribution protocols. OSPF and ISIS are IGPs and do not distribute labels, although they can be used for SR, but SR still relies on IGP to advertise prefix-SIDs, not as a label distribution protocol per se.
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.
✗
OSPF
Why it's wrong here
OSPF is an IGP and does not distribute MPLS labels; it can advertise segment routing SIDs but not perform label distribution.
✓
LDP
Why this is correct
LDP is the most common label distribution protocol for MPLS.
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.
Trap categories for this question
Similar concept trap
IS-IS is an IGP and similarly does not distribute MPLS labels directly.
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 350-501 OSPF questions on adjacency and route selection.
Architecture — This question tests Architecture — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: LDP — Options A, B, and C are correct. LDP, BGP, and RSVP are standard label distribution protocols. OSPF and ISIS are IGPs and do not distribute labels, although they can be used for SR, but SR still relies on IGP to advertise prefix-SIDs, not as a label distribution protocol per se.
What should I do if I get this 350-501 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 350-501 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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Question Discussion
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