The answer is that the ingress LSR imposes a label on the packet, and a transit LSR performs label swapping. This is correct because in MPLS label switching operations, the ingress Label Switch Router (LSR) pushes a new label onto the unlabeled packet to begin the label-switched path, while each transit LSR swaps the incoming label for an outgoing label from its LFIB, forwarding the packet toward the egress. On the Cisco SPCOR 350-501 exam, this concept tests your understanding of the fundamental push, swap, pop, and PHP operations, often appearing in a "choose two" format where a common trap is confusing the egress LSR’s pop with the transit LSR’s swap. Remember that PHP (Penultimate Hop Popping) is a special case where the penultimate LSR pops the label before the egress, but the core rule remains: ingress pushes, transit swaps, egress pops. A helpful memory tip is "Push at the start, Swap in the heart, Pop at the end—PHP lets the penultimate send."
350-501 MPLS and Segment Routing Practice Question
This 350-501 practice question tests your understanding of mpls and segment routing. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
Which TWO statements about MPLS label switching are correct? (Choose two.)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The transit LSR performs label swapping.
Option A is correct because a transit Label Switch Router (LSR) in an MPLS network performs label swapping: it receives a labeled packet, replaces the incoming label with an outgoing label from its LFIB (Label Forwarding Information Base), and forwards the packet toward the egress LSR. This is the fundamental operation of an LSR in the core of an MPLS domain, as 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 transit LSR performs label swapping.
Why this is correct
Correct: Transit routers swap the incoming label with an outgoing label.
Related concept
Read the scenario before looking for a memorised answer.
✗
The CE receives a frame with an MPLS label.
Why it's wrong here
CE receives an IP packet without label (unless configured for MPLS).
✓
The ingress LSR imposes a label on the packet.
Why this is correct
Correct: Ingress does label imposition (push).
Related concept
Read the scenario before looking for a memorised answer.
✗
PHP (Penultimate Hop Popping) causes the egress router to pop the label.
Why it's wrong here
PHP causes the penultimate hop to pop the label, not the egress.
✗
The egress LSR performs label swapping before forwarding.
Why it's wrong here
Egress pops the label, not swap.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between which router performs label popping in PHP (penultimate hop vs. egress) and which router swaps labels (transit LSR vs. egress LSR), leading candidates to confuse the roles of the penultimate and egress routers.
Detailed technical explanation
How to think about this question
In MPLS, label operations are defined by the LSR role: ingress LSR imposes (push) a label, transit LSR swaps labels, and egress LSR pops the label. PHP is an optimization where the penultimate LSR pops the label so the egress LSR does not need to perform a label lookup, reducing its processing load. The LFIB on each LSR contains the exact label operation (swap, push, or pop) based on the FEC (Forwarding Equivalence Class).
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.
MPLS and Segment Routing — This question tests MPLS and Segment Routing — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The transit LSR performs label swapping. — Option A is correct because a transit Label Switch Router (LSR) in an MPLS network performs label swapping: it receives a labeled packet, replaces the incoming label with an outgoing label from its LFIB (Label Forwarding Information Base), and forwards the packet toward the egress LSR. This is the fundamental operation of an LSR in the core of an MPLS domain, as 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 →
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Which two statements about MPLS label operations are true? (Choose two.)
easy
A.Push operation can only add one label at a time.
B.Pop operation can remove multiple labels at once.
✓ C.Push operation adds a label to the packet.
D.Swap operation removes and replaces two labels.
✓ E.Pop operation removes the top label and may be triggered by implicit-null.
Why C: A correct: push is adding a label. B correct: pop is removing exactly one label, and PHP uses implicit-null. C wrong: swap replaces label. D wrong: push adds one or more. E wrong: pop removes exactly one, not multiple at once (except in PHP).
Last reviewed: Jun 11, 2026
Question Discussion
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