The answer is that the router has a directly connected neighbor which is the egress LER, making the implicit-null label (label 3) a normal and expected part of Penultimate Hop Popping (PHP). When a P router sees only implicit-null labels for certain destinations, it means the next-hop router is directly connected and has advertised label 3 to trigger PHP, so the P router pops the label before forwarding the packet to the egress LER. On the Cisco SPCOR 350-501 exam, this concept tests your understanding of MPLS label operations and the distinction between implicit-null (label 3) and explicit-null (label 0), with the common trap being to mistake implicit-null for a misconfiguration or error. Remember that implicit-null is the default behavior for directly connected egress routers, not a fault. A helpful memory tip: "Implicit-null means 'I'm the last hop, pop it off'—think of it as the router saying 'I'm done with labels, just send the packet raw.'"
350-501 MPLS and Segment Routing Practice Question
This 350-501 practice question tests your understanding of mpls and segment routing. 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 is troubleshooting MPLS label switching in a service provider core. They notice that packets are being forwarded correctly between provider edge routers, but when they check the MPLS forwarding table on a P router, they see only implicit-null labels for some destinations. What is the most likely reason for this?
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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The router has a directly connected neighbor that is the egress LER.
Implicit-null (label 3) is advertised when the next-hop router is directly connected and wants to trigger PHP. This is normal behavior. Option B is incorrect because implicit-null is not a sign of misconfiguration. Option C is incorrect because explicit-null is label 0. Option D is incorrect because PHP is standard.
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 router is using explicit-null label due to security policies.
Why it's wrong here
Explicit-null is label 0, used for QoS preservation, not security.
✓
The router has a directly connected neighbor that is the egress LER.
Why this is correct
Implicit-null is used in PHP; the egress LER advertises it to the penultimate hop.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The router is misconfigured to use implicit-null for all labels.
Why it's wrong here
Implicit-null is a standard behavior, not a misconfiguration.
✗
The router is performing penultimate hop popping (PHP) incorrectly.
Why it's wrong here
PHP is correct; implicit-null indicates proper PHP operation.
Common exam traps
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.
MPLS and Segment Routing — This question tests MPLS and Segment Routing — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The router has a directly connected neighbor that is the egress LER. — Implicit-null (label 3) is advertised when the next-hop router is directly connected and wants to trigger PHP. This is normal behavior. Option B is incorrect because implicit-null is not a sign of misconfiguration. Option C is incorrect because explicit-null is label 0. Option D is incorrect because PHP is standard.
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: "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?
OSPF neighbours must agree on key parameters.
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