The answer is that LDP is not enabled on the interfaces. This is the most likely cause because MPLS LDP requires explicit interface-level activation using the `mpls ip` command; even when the global LDP process is configured and OSPF neighbor adjacencies are fully established, LDP sessions will not form on interfaces where this command is missing. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding that LDP interface activation is a separate step from global configuration—a common trap where candidates assume OSPF adjacency guarantees LDP operation. The key distinction is that OSPF runs at Layer 3, while LDP needs per-interface permission to exchange label bindings. Memory tip: think "OSPF up does not mean LDP up—you must `mpls ip` each interface."
350-501 Architecture Practice Question
This 350-501 practice question tests your understanding of architecture. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
Exhibit
Refer to the exhibit.
interface GigabitEthernet0/0
ip address 10.0.0.1 255.255.255.252
mpls ip
mpls mtu 1500
!
interface GigabitEthernet0/1
ip address 10.0.0.5 255.255.255.252
mpls ip
mpls mtu 1500
!
router ospf 1
network 10.0.0.0 0.0.0.3 area 0
network 10.0.0.4 0.0.0.3 area 0
!
mpls ldp router-id Loopback0 force
!
mpls label range 100 199
!
mpls label protocol ldp
Refer to the exhibit. An engineer configures MPLS LDP on a router. The router has two interfaces with IP addresses 10.0.0.1/30 and 10.0.0.5/30. The engineer notices that LDP sessions are not established. The OSPF neighbor adjacencies are up. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Refer to the exhibit.
interface GigabitEthernet0/0
ip address 10.0.0.1 255.255.255.252
mpls ip
mpls mtu 1500
!
interface GigabitEthernet0/1
ip address 10.0.0.5 255.255.255.252
mpls ip
mpls mtu 1500
!
router ospf 1
network 10.0.0.0 0.0.0.3 area 0
network 10.0.0.4 0.0.0.3 area 0
!
mpls ldp router-id Loopback0 force
!
mpls label range 100 199
!
mpls label protocol ldp
A
The OSPF network statements do not cover the interfaces correctly
Why wrong: OSPF adjacencies are up, so OSPF is working.
B
The MPLS MTU is set to 1500, which is too low
Why wrong: MTU 1500 is standard and should work.
C
The label range is too small
Why wrong: Label range of 100-199 is sufficient for two interfaces.
D
LDP is not enabled on the interfaces
The 'mpls ip' command does not enable LDP; LDP requires 'mpls ldp' or explicit configuration.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
LDP is not enabled on the interfaces
D is correct because MPLS LDP requires explicit interface-level activation under the `mpls ip` command. Even if OSPF adjacencies are up and the global LDP process is configured, LDP will not form sessions on interfaces where `mpls ip` is missing. The engineer likely enabled LDP globally but forgot to enable it on the specific interfaces, which is a common oversight.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 OSPF network statements do not cover the interfaces correctly
Label range of 100-199 is sufficient for two interfaces.
✓
LDP is not enabled on the interfaces
Why this is correct
The 'mpls ip' command does not enable LDP; LDP requires 'mpls ldp' or explicit configuration.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between global LDP configuration and interface-level activation, leading candidates to assume that enabling LDP globally is sufficient for session establishment.
Detailed technical explanation
How to think about this question
LDP uses UDP hello messages (port 646) to discover neighbors and then establishes a TCP session (also port 646) for label exchange. The `mpls ip` interface command enables both the sending of LDP hellos and the processing of incoming hellos on that interface. Without it, the router will not participate in LDP neighbor discovery, even if OSPF is fully operational. In real-world deployments, this is often missed when adding new interfaces to an existing MPLS network.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Architecture — This question tests Architecture — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: LDP is not enabled on the interfaces — D is correct because MPLS LDP requires explicit interface-level activation under the `mpls ip` command. Even if OSPF adjacencies are up and the global LDP process is configured, LDP will not form sessions on interfaces where `mpls ip` is missing. The engineer likely enabled LDP globally but forgot to enable it on the specific interfaces, which is a common oversight.
What should I do if I get this 350-501 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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