- A
Penultimate Hop Popping (PHP) reduces the processing load on the egress LSR by having the penultimate LSR pop the label.
Correct. PHP offloads the label removal to the penultimate LSR, reducing egress LSR work.
- B
An intermediate LSR performs a label swap operation: it replaces the incoming label with an outgoing label.
Correct. In the MPLS data plane, intermediate LSRs swap the top label based on the LFIB.
- C
The egress LSR performs label imposition (push) before forwarding the IP packet to the destination.
Why wrong: Incorrect. The egress LSR performs label disposition (pop), not imposition. Imposition occurs at the ingress.
- D
PHP is enabled only when the egress LSR is not directly connected to the penultimate LSR.
Why wrong: Incorrect. PHP is typically used when the egress LSR is directly connected; it is the default behavior.
- E
By default, PHP is disabled on Cisco IOS routers and must be explicitly configured.
Why wrong: Incorrect. PHP is enabled by default for directly connected egress LSRs; no explicit configuration is needed.
300-410 MPLS Operations Practice Question
This 300-410 practice question tests your understanding of mpls operations. 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 imposition, disposition, and swapping are true? (Choose TWO.)
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
Penultimate Hop Popping (PHP) reduces the processing load on the egress LSR by having the penultimate LSR pop the label.
In MPLS forwarding, label imposition (push) occurs at the ingress LSR, disposition (pop) at the egress LSR, and swapping (swap) at intermediate LSRs. The PHP (Penultimate Hop Popping) feature causes the penultimate LSR to pop the label before the egress LSR receives the packet, so the egress LSR sees only the IP packet. Option A is correct: PHP reduces the load on the egress LSR. Option B is correct: an intermediate LSR typically swaps the top label. Option C is false: the egress LSR does not impose a label; it removes it. Option D is false: PHP is the default behavior for directly connected egress LSRs. Option E is false: PHP is not disabled by default; it is enabled.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Penultimate Hop Popping (PHP) reduces the processing load on the egress LSR by having the penultimate LSR pop the label.
Why this is correct
Correct. PHP offloads the label removal to the penultimate LSR, reducing egress LSR work.
Related concept
Static NAT maps one inside address to one outside address.
- ✓
An intermediate LSR performs a label swap operation: it replaces the incoming label with an outgoing label.
Why this is correct
Correct. In the MPLS data plane, intermediate LSRs swap the top label based on the LFIB.
Related concept
Static NAT maps one inside address to one outside address.
- ✗
The egress LSR performs label imposition (push) before forwarding the IP packet to the destination.
Why it's wrong here
Incorrect. The egress LSR performs label disposition (pop), not imposition. Imposition occurs at the ingress.
- ✗
PHP is enabled only when the egress LSR is not directly connected to the penultimate LSR.
Why it's wrong here
Incorrect. PHP is typically used when the egress LSR is directly connected; it is the default behavior.
- ✗
By default, PHP is disabled on Cisco IOS routers and must be explicitly configured.
Why it's wrong here
Incorrect. PHP is enabled by default for directly connected egress LSRs; no explicit configuration is needed.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
- Static NAT maps one inside address to one outside address.
- PAT allows many inside hosts to share one public address using ports.
- Inside local and inside global describe the private and translated addresses.
- NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
- Identify inside and outside interfaces first.
- Check whether the scenario needs static NAT, dynamic NAT or PAT.
- Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
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FAQ
Questions learners often ask
What does this 300-410 question test?
MPLS Operations — This question tests MPLS Operations — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: Penultimate Hop Popping (PHP) reduces the processing load on the egress LSR by having the penultimate LSR pop the label. — In MPLS forwarding, label imposition (push) occurs at the ingress LSR, disposition (pop) at the egress LSR, and swapping (swap) at intermediate LSRs. The PHP (Penultimate Hop Popping) feature causes the penultimate LSR to pop the label before the egress LSR receives the packet, so the egress LSR sees only the IP packet. Option A is correct: PHP reduces the load on the egress LSR. Option B is correct: an intermediate LSR typically swaps the top label. Option C is false: the egress LSR does not impose a label; it removes it. Option D is false: PHP is the default behavior for directly connected egress LSRs. Option E is false: PHP is not disabled by default; it is enabled.
What should I do if I get this 300-410 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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Last reviewed: Jun 18, 2026
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