Quick Answer
The correct first step is to verify LDP neighbor adjacency. This is because an MPLS LSP relies on Label Distribution Protocol (LDP) to exchange labels between directly connected peers; without a stable LDP adjacency, label bindings cannot be distributed, and the LSP will fail to establish. On the Cisco SPCOR 350-501 exam, this question tests your understanding of the MPLS control plane hierarchy—many candidates mistakenly jump to checking BGP or OSPF, but LDP adjacency is the foundational prerequisite for any label-switched path. A common trap is assuming that reachability (ping) confirms the LSP, but ping only tests IP forwarding, not label switching. For a quick memory tip, remember the LSP troubleshooting mantra: “Labels first, routing second”—always confirm LDP adjacency before checking IGP or BGP.
This 350-501 practice question tests your understanding of architecture. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
An engineer notices that an MPLS LSP in the core is not establishing. Which troubleshooting step should be performed first?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Verify LDP neighbor adjacency
LDP adjacency is essential for label exchange and LSP building.
Ping the far-end loopback interface
Why wrong: Ping tests reachability, not LSP.
Check BGP IPv4 unicast neighbors
Why wrong: BGP is not required for LSP establishment.
Check OSPF neighbor adjacency on all links
Why wrong: OSPF may not be used for MPLS.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Verify LDP neighbor adjacency
Option B is correct because verifying LDP adjacency is the first step in troubleshooting MPLS LSP establishment. Option A is wrong because checking BGP is not directly relevant to MPLS LSP establishment. Option C is wrong because pinging the far-end loopback tests reachability but not LSP establishment. Option D is wrong because checking OSPF neighbor adjacency is not specific to MPLS.
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.
Verify LDP neighbor adjacency
Why this is correct
LDP adjacency is essential for label exchange and LSP building.
Clue confirmation
The clue word "first" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
Ping the far-end loopback interface
Why it's wrong here
Ping tests reachability, not LSP.
Check BGP IPv4 unicast neighbors
Why it's wrong here
BGP is not required for LSP establishment.
Check OSPF neighbor adjacency on all links
Why it's wrong here
OSPF may not be used for MPLS.
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.
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FAQ
Architecture — This question tests Architecture — OSPF neighbours must agree on key parameters..
The correct answer is: Verify LDP neighbor adjacency — Option B is correct because verifying LDP adjacency is the first step in troubleshooting MPLS LSP establishment. Option A is wrong because checking BGP is not directly relevant to MPLS LSP establishment. Option C is wrong because pinging the far-end loopback tests reachability but not LSP establishment. Option D is wrong because checking OSPF neighbor adjacency is not specific to MPLS.
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.
Yes — watch for: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 24, 2026
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