The answer is that the 'Pop tag' in the outgoing tag indicates the router will remove the MPLS label before forwarding the packet. This is correct because the router is acting as the penultimate hop in the LSP, performing Penultimate Hop Popping (PHP) behavior as defined in RFC 3031. By popping the label, the egress LSR receives a pure IP packet and avoids an extra label lookup, optimizing forwarding efficiency. On the Cisco SPCOR 350-501 exam, this concept tests your understanding of MPLS label operations, specifically how PHP and explicit label assignments interact at the edge of the service provider core. A common trap is confusing the penultimate hop with the egress router—remember that the penultimate hop pops, while the egress router only sees an IP packet. Memory tip: "PHP = Pop Here, Please" — the penultimate hop does the work so the egress can rest.
350-501 Services Practice Question
This 350-501 practice question tests your understanding of services. 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.
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?
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
A
The router will swap the label with the local label 16.
Why wrong: Local tag is the incoming label, not outgoing.
B
The router has received an error in label distribution.
Why wrong: PHP is normal behavior.
C
The router will pop the MPLS label before forwarding the packet.
PHP is performed by the penultimate router.
D
The router will forward the packet without an MPLS label.
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.
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 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.
Related concept
Read the scenario before looking for a memorised answer.
✗
The router will forward the packet without an MPLS label.
Why it's wrong here
That is indicated by 'Untagged'.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Detailed technical explanation
How to think about this question
In MPLS, the penultimate hop popping (PHP) mechanism is negotiated via LDP (Label Distribution Protocol) using the 'Implicit NULL' label (value 3). When the egress LSR advertises an Implicit NULL for a FEC, the penultimate router knows to pop the label. This reduces the forwarding burden on the egress LSR, which would otherwise have to perform two lookups (label and IP). In real-world deployments, PHP is enabled by default on most Cisco IOS and IOS-XE platforms, and 'show mpls forwarding-table' displays 'Pop tag' for such entries.
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 practitioner preparing for the 350-501 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
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.
Services — This question tests Services — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
B.The outgoing interface uses penultimate hop popping.
C.The outgoing label is Untagged.
D.The local label is 20.
Why A: Option A is correct because the forwarding entry for 10.2.2.0/24 shows an outgoing label of 20, which means the egress LSR is not performing penultimate hop popping (PHP). In MPLS, when the outgoing label is not the implicit-null label (3) or explicit-null label (0), the penultimate hop must push that label, and the egress LSR will perform a full label lookup. This is an explicit label request, not PHP.
Last reviewed: Jun 11, 2026
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
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.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.