Quick Answer
The correct answer is LDP, as it is the standard protocol for MPLS label distribution in a dynamic MPLS network. LDP automatically establishes Label Switched Paths (LSPs) by exchanging label bindings between directly connected peers, enabling all routers in the MPLS domain to dynamically learn transport labels without manual configuration. On the Cisco SPCOR 350-501 exam, this concept tests your understanding of the MPLS control plane fundamentals, often appearing in questions that distinguish LDP from RSVP-TE (used for traffic engineering) or iBGP (which carries VPN labels, not transport labels). A common trap is confusing LDP with OSPF, but OSPF is an IGP that builds the routing table, not a label distribution protocol. Remember the memory tip: “LDP for Labels, RSVP for Reservations, iBGP for VPNs”—this helps you quickly eliminate wrong choices when asked about basic dynamic MPLS setup.
This 350-501 practice question tests your understanding of mpls and segment routing. 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.
A service provider is deploying MPLS in their core network and wants to ensure that all routers in the MPLS domain can dynamically exchange label bindings. Which protocol should be enabled on all routers to meet this requirement?
LDP
LDP dynamically distributes labels for all prefixes in the IGP.
OSPF
Why wrong: OSPF is an IGP that does not distribute MPLS labels.
iBGP
Why wrong: iBGP carries VPNv4 routes and labels for Layer 3 VPNs, not transport labels.
RSVP-TE
Why wrong: RSVP-TE is used for traffic engineering tunnels, not for general label distribution.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
LDP
LDP is the standard protocol for distributing MPLS labels in a dynamic MPLS network. Option B is incorrect because RSVP-TE is used for traffic engineering, not basic label distribution. Option C is incorrect because iBGP carries VPN labels, not transport labels. Option D is incorrect because OSPF does not distribute labels.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
LDP
Why this is correct
LDP dynamically distributes labels for all prefixes in the IGP.
Related concept
OSPF neighbours must agree on key parameters.
OSPF
Why it's wrong here
OSPF is an IGP that does not distribute MPLS labels.
iBGP
Why it's wrong here
iBGP carries VPNv4 routes and labels for Layer 3 VPNs, not transport labels.
RSVP-TE
Why it's wrong here
RSVP-TE is used for traffic engineering tunnels, not for general label distribution.
Common exam traps
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
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.
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
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
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.
MPLS and Segment Routing — study guide chapter
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MPLS and Segment Routing practice questions
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FAQ
MPLS and Segment Routing — This question tests MPLS and Segment Routing — OSPF neighbours must agree on key parameters..
The correct answer is: LDP — LDP is the standard protocol for distributing MPLS labels in a dynamic MPLS network. Option B is incorrect because RSVP-TE is used for traffic engineering, not basic label distribution. Option C is incorrect because iBGP carries VPN labels, not transport labels. Option D is incorrect because OSPF does not distribute labels.
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.
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 24, 2026
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