350-501 MPLS and Segment Routing Practice Question
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?
⚠ Common exam trap
Test-takers frequently confuse implicit-null with a misconfiguration or error, when in fact it is a normal and expected result of PHP being triggered by a directly connected egress LER.
Answer choices
Why each option matters
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.
When a P router sees only implicit-null labels (label 3) for some destinations in its MPLS forwarding table, it indicates that the router is performing penultimate hop popping (PHP). This occurs when the P router is the penultimate hop before the egress Label Edge Router (LER). The egress LER advertises an implicit-null label to its directly connected neighbor, causing the P router to pop the label and forward the packet using only the IP header. Option B is correct because the P router has a directly connected neighbor that is the egress LER, which triggers PHP.
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.
- ✗
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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