Question 2,052 of 2,152

MPLS Operations →mediumMultiple ChoiceObjective-mapped

MPLS TTL Propagation Feature: Purpose and Behavior

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.

In MPLS, what is the purpose of the TTL propagation feature?

A
It propagates the MPLS label stack to all routers in the domain.
Why wrong: TTL propagation is about TTL values, not label stack propagation.
B
It copies the IP TTL into the MPLS TTL at the ingress LER, allowing traceroute to see each hop in the MPLS path.
When enabled, the IP TTL is decremented and copied to the MPLS label, so each LSR decrements the TTL, making traceroute visible.
C
It prevents TTL from being decremented in the MPLS core for security purposes.
Why wrong: Disabling TTL propagation prevents decrementing, but that is the opposite of propagation.
D
It ensures that the MPLS TTL is always equal to the IP TTL at the egress LER.
Why wrong: At the egress, the MPLS TTL is copied back to IP TTL, but propagation refers to ingress behavior.

Full breakdown with real-world context →

Quick Answer

The purpose of the MPLS TTL propagation feature is to copy the IP TTL value into the MPLS TTL at the ingress Label Edge Router (LER), which allows standard traceroute to reveal every hop along the MPLS path. This behavior is critical because traceroute relies on TTL expiration to generate ICMP time-exceeded messages; without propagation, the MPLS core would appear as a single hop, breaking network troubleshooting. On the Cisco CCNP ENARSI 300-410 exam, this concept tests your understanding of how MPLS interacts with IP tools and is often presented in a scenario where you must decide whether to enable or disable propagation for security or diagnostic purposes. A common trap is assuming that disabling propagation only affects traceroute, but it also changes how the MPLS TTL is set—defaulting to 255 instead of copying the IP TTL. For a memory tip, think of “propagation equals transparency”: when enabled, the core is visible; when disabled, the core is hidden.

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

✓

It copies the IP TTL into the MPLS TTL at the ingress LER, allowing traceroute to see each hop in the MPLS path.

The TTL propagation feature controls how the IP TTL is handled when packets enter an MPLS domain. By default, the IP TTL is copied to the MPLS TTL (propagation), allowing traceroute to work across the MPLS network. When disabled, the MPLS TTL is set to 255, hiding the core from traceroute.

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.

✗
It propagates the MPLS label stack to all routers in the domain.
Why it's wrong here
TTL propagation is about TTL values, not label stack propagation.
✓
It copies the IP TTL into the MPLS TTL at the ingress LER, allowing traceroute to see each hop in the MPLS path.
Why this is correct
When enabled, the IP TTL is decremented and copied to the MPLS label, so each LSR decrements the TTL, making traceroute visible.
Related concept
Read the scenario before looking for a memorised answer.
✗
It prevents TTL from being decremented in the MPLS core for security purposes.
Why it's wrong here
Disabling TTL propagation prevents decrementing, but that is the opposite of propagation.
✗
It ensures that the MPLS TTL is always equal to the IP TTL at the egress LER.
Why it's wrong here
At the egress, the MPLS TTL is copied back to IP TTL, but propagation refers to ingress behavior.

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 300-410 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 300-410 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.

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FAQ

Questions learners often ask

What does this 300-410 question test?

MPLS Operations — This question tests MPLS Operations — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: It copies the IP TTL into the MPLS TTL at the ingress LER, allowing traceroute to see each hop in the MPLS path. — The TTL propagation feature controls how the IP TTL is handled when packets enter an MPLS domain. By default, the IP TTL is copied to the MPLS TTL (propagation), allowing traceroute to work across the MPLS network. When disabled, the MPLS TTL is set to 255, hiding the core from traceroute.

What should I do if I get this 300-410 question wrong?

What is the key concept behind this question?

Read the scenario before looking for a memorised answer.

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