The answer is to verify that the interface has 'mpls ip' enabled. This is the most fundamental prerequisite for LDP operation because, even after enabling LDP globally with 'mpls ldp autoconfig' or 'mpls ip' at the interface level, the session will not initiate unless the interface itself is explicitly configured to run MPLS. Without this command, the router will not send or process LDP hello messages on that link, so the neighbor discovery phase never begins. On the Cisco SPCOR 350-501 exam, this question tests your ability to distinguish between global configuration and interface-level activation—a common trap where candidates assume global LDP enablement is sufficient. Remember that OSPF adjacency may be up, but MPLS requires its own interface trigger. A useful memory tip is "global for the protocol, interface for the packets"—LDP needs both layers to establish.
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.
A network engineer is configuring MPLS LDP on a new router. After enabling LDP globally and on the interface, the LDP session does not establish. The IGP (OSPF) is fully operational. What should the engineer verify 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The interface has 'mpls ip' enabled.
Option A is correct because the most basic check is whether the interface has 'mpls ip' enabled. Option B is wrong because LDP router-id can be any reachable IP, not necessarily a loopback. Option C is wrong because password mismatch would cause authentication failure, but the session might still attempt to establish. Option D is wrong because IGP metric does not affect LDP session establishment.
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.
✗
The LDP password matches.
Why it's wrong here
While password mismatch can prevent session establishment, it is not the first thing to check if no authentication is configured.
✗
The router-id is a loopback.
Why it's wrong here
LDP can use any IP address as router-id, and a loopback is not mandatory.
✗
The IGP metric is set.
Why it's wrong here
IGP metric does not affect LDP session establishment; LDP uses IGP to resolve transport addresses, but metric is irrelevant.
✓
The interface has 'mpls ip' enabled.
Why this is correct
This is the most basic verification; if MPLS is not enabled on the interface, LDP will not form.
Clue confirmation
The clue word "first" in the question point toward this answer.
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 350-501 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: The interface has 'mpls ip' enabled. — Option A is correct because the most basic check is whether the interface has 'mpls ip' enabled. Option B is wrong because LDP router-id can be any reachable IP, not necessarily a loopback. Option C is wrong because password mismatch would cause authentication failure, but the session might still attempt to establish. Option D is wrong because IGP metric does not affect LDP session establishment.
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.
Are there clue words in this question I should notice?
Yes — watch for: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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