mediumMultiple Choice
300-410 Practice Question: In MPLS, what is the purpose of the TTL…
In MPLS, what is the purpose of the TTL propagation feature?
⚠ Common exam trap
The trap is confusing TTL propagation with label propagation; candidates may think it distributes labels, but it specifically deals with TTL copying for traceroute visibility.
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.
TTL propagation in MPLS controls whether the IP TTL is copied into the MPLS TTL field at the ingress LER. When enabled (default on Cisco), the ingress copies the IP TTL to the MPLS TTL, and each LSR decrements it. This allows traceroute to reveal each hop in the MPLS core. When disabled, the MPLS TTL is set to 255 and not propagated, 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 concerns the time-to-live field only; label stack distribution is handled by LDP or RSVP-TE signalling. It is tempting because propagation sounds like distributing information across the domain, which is what label distribution protocols genuinely do when establishing LSPs between routers.
- ✓
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
With TTL propagation enabled, the ingress LER copies the IP TTL value into the imposed MPLS header TTL, so each transit LSR decrements it and ICMP time-exceeded messages reveal every hop. This satisfies the requirement that traceroute display each hop along the MPLS path.
- ✗
It prevents TTL from being decremented in the MPLS core for security purposes.
Why it's wrong here
TTL propagation controls whether the TTL field is copied into the label stack header, allowing traceroute to reveal core hops; it is not a security mechanism and does not stop decrementing. It is tempting because disabling propagation hides core topology, but that is a side effect, not the feature's purpose.
- ✗
It ensures that the MPLS TTL is always equal to the IP TTL at the egress LER.
Why it's wrong here
TTL propagation copies the IP TTL into the MPLS header at ingress and back at egress; it does not force equality at the egress LER, since the label TTL decrements per hop independently. It is tempting because propagation does synchronise TTL values at the ingress edge, which is where the copying actually happens.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.