The correct answer is that the penultimate hop pops the top label before forwarding to the egress router, while the ingress router performs a push operation to add the label stack. This is correct because in MPLS label operations, the ingress Label Edge Router (LER) pushes a label onto the incoming IP packet to establish a Label Switched Path (LSP) through the core, and the penultimate hop router (the router just before the egress) performs a penultimate hop popping (PHP) to remove the top label, so the egress router only needs to perform a simple IP lookup without processing an MPLS label. On the Cisco SPCOR 350-501 exam, this concept tests your understanding of MPLS forwarding mechanics and is a common trap where candidates mistakenly think the egress router always pops the label. A helpful memory tip is "PHP: Pop Here, Please" — the penultimate hop does the final pop so the egress can exit quickly.
350-501 Architecture Practice Question
This 350-501 practice question tests your understanding of architecture. 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 operations in a service provider core 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 ingress router pushes a label stack onto the IP packet
Option A is correct because the ingress router (LER) in an MPLS network performs a push operation, adding a label stack (typically one or more labels) onto the incoming IP packet. This label stack is used to direct the packet along a Label Switched Path (LSP) through the core, enabling MPLS forwarding based on labels rather than IP routing.
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 ingress router pushes a label stack onto the IP packet
Why this is correct
Ingress pushes the label stack to encapsulate the packet.
Related concept
Read the scenario before looking for a memorised answer.
✗
The egress router receives an MPLS packet with two labels by default
Why it's wrong here
After PHP, the egress receives a single label or IP packet.
✗
The penultimate hop performs label swap for the top label
Why it's wrong here
Penultimate hop performs label pop (PHP), not swap.
✓
The penultimate hop pops the top label before forwarding to the egress router
Why this is correct
PHP removes the top label to reduce egress processing.
Related concept
Read the scenario before looking for a memorised answer.
✗
The penultimate hop is disabled by default in MPLS networks
Why it's wrong here
PHP is enabled by default in most implementations.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that the penultimate hop performs a label swap, when in fact it performs a pop (PHP) by default, and that the egress router always receives two labels, which is only true in specific scenarios like MPLS VPNs with a transport label and a VPN label.
Detailed technical explanation
How to think about this question
In MPLS, the penultimate hop popping (PHP) mechanism is defined in RFC 3031, where the penultimate router removes the top label before forwarding the packet to the egress router, reducing the egress router's processing load. The egress router then receives either a native IP packet or a packet with an explicit null label (label value 0 or 2) if PHP is disabled. This behavior is critical for optimizing performance in high-speed core networks.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
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.
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: The ingress router pushes a label stack onto the IP packet — Option A is correct because the ingress router (LER) in an MPLS network performs a push operation, adding a label stack (typically one or more labels) onto the incoming IP packet. This label stack is used to direct the packet along a Label Switched Path (LSP) through the core, enabling MPLS forwarding based on labels rather than IP routing.
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.
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Question Discussion
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