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300-410 Practice Question: In MPLS, what is the purpose of the TTL…
In MPLS, what is the purpose of the TTL propagation feature?
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.
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.
- ✗
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.
Go deeper
Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
Key term
LDP Protocol
LDP, or Label Distribution Protocol, is a protocol that routers use to automatically exchange labels that enable MPLS (Multiprotocol Label Switching) to create fast, efficient paths for data packets across a network.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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