The answer is that the ingress PE pushes a new label onto the packet. This is correct because in an MPLS core running LDP, the ingress Provider Edge router must assign and insert an outer label for the Forwarding Equivalence Class (FEC) before the packet enters the label-switched path. The push operation creates the initial label stack, enabling subsequent transit LSRs to perform label swapping rather than imposing labels themselves. On the Cisco SPCOR 350-501 exam, this concept tests your understanding of MPLS label operations at different roles: ingress (push), transit (swap), and penultimate hop (pop). A common trap is confusing the ingress push with the PHP pop—remember that only the ingress router pushes a new label; the penultimate hop pops it. Memory tip: "Ingress Imposes, Egress Exits, Transit Trades."
350-501 Architecture Practice Question
This 350-501 practice question tests your understanding of architecture. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
A service provider is deploying MPLS in its core network. The core routers are all configured with LDP. Which label operation does an ingress PE perform on the first packet of a new flow?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Push a new label onto the packet
The ingress PE pushes a new label onto the packet. Option B is correct because LDP assigns labels for each FEC, and the ingress PE pushes an outer label. Option A is wrong because swapping is done at transit LSRs. Option C is wrong because PHP results in label removal at the penultimate hop. Option D is wrong because pop is the same as removal.
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.
✓
Push a new label onto the packet
Why this is correct
The ingress PE pushes the label corresponding to the FEC for the destination.
Clue confirmation
The clue word "first" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Remove the label stack entirely
Why it's wrong here
Removal of the entire label stack is done at the egress PE.
✗
Pop the label (PHP)
Why it's wrong here
PHP happens at the penultimate hop to reduce load on the egress PE.
✗
Swap the incoming label for an outgoing label
Why it's wrong here
Swapping is performed at intermediate LSRs, not at the ingress PE.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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 350-501 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Architecture — This question tests Architecture — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Push a new label onto the packet — The ingress PE pushes a new label onto the packet. Option B is correct because LDP assigns labels for each FEC, and the ingress PE pushes an outer label. Option A is wrong because swapping is done at transit LSRs. Option C is wrong because PHP results in label removal at the penultimate hop. Option D is wrong because pop is the same as removal.
What should I do if I get this 350-501 question wrong?
Identify which 350-501 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Are there clue words in this question I should notice?
Yes — watch for: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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