350-501 Services Practice Question
Exhibit
Router# show mpls forwarding-table 10.10.10.0/24 Local tag Outgoing tag Prefix Outgoing interface Bytes tag switched 16 Pop tag 10.10.10.0/24 GigabitEthernet0/0/0/0 345678 17 Untagged 10.10.10.0/24 GigabitEthernet0/0/0/1 123456
Refer to the exhibit. A network engineer is troubleshooting MPLS forwarding for prefix 10.10.10.0/24. The router shows two entries. What does the 'Pop tag' in the outgoing tag indicate?
⚠ Common exam trap
Cisco often tests the distinction between 'Pop tag' (PHP/Implicit NULL) and 'Untagged' (no label at all), leading candidates to confuse the two; 'Pop tag' means the label is actively removed, while 'Untagged' means the packet was never labeled.
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 will pop the MPLS label before forwarding the packet.
The 'Pop tag' in the outgoing tag indicates that the router is the penultimate hop in the MPLS LSP. According to Penultimate Hop Popping (PHP) behavior, the penultimate router removes (pops) the top label before forwarding the IP packet to the egress LSR, so the egress router receives a pure IP packet and does not need to perform a label lookup. This is standard MPLS behavior defined in RFC 3031.
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 will swap the label with the local label 16.
Why it's wrong here
Local tag is the incoming label, not outgoing.
- ✗
The router has received an error in label distribution.
Why it's wrong here
PHP is normal behavior.
- ✓
The router will pop the MPLS label before forwarding the packet.
Why this is correct
PHP is performed by the penultimate router.
- ✗
The router will forward the packet without an MPLS label.
Why it's wrong here
That is indicated by 'Untagged'.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-501 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-501 exam.