hardMultiple Select
300-410 Practice Question: Which TWO statements correctly describe the…
Which TWO statements correctly describe the behavior of TTL propagation in MPLS networks? (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
✓
Disabling TTL propagation prevents traceroute from revealing the internal LSR hops.
In MPLS, TTL propagation can be disabled for security or to hide the core topology. When disabled, the IP TTL is copied to the MPLS label only at the ingress LSR, and at the egress LSR the MPLS TTL is copied back to the IP header. By default, TTL propagation is enabled. The command 'no mpls ip propagate-ttl' disables it. Option A is correct because disabling TTL propagation hides the core hops from traceroute. Option B is correct because the command to disable is indeed 'no mpls ip propagate-ttl'. Option C is false: the command is not 'no mpls ip ttl-propagate'. Option D is false: traceroute shows only the ingress and egress LSRs, not all hops. Option E is false: when disabled, the TTL is not decremented across the MPLS core.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Disabling TTL propagation prevents traceroute from revealing the internal LSR hops.
Why this is correct
Disabling TTL propagation stops the ingress LSR copying the IP TTL into the MPLS header, so transit LSRs decrement a value that never reaches the traceroute probe. Internal hops therefore appear as a single MPLS cloud, satisfying the requirement to conceal provider core topology from customers.
- ✓
The command 'no mpls ip propagate-ttl' is used to disable TTL propagation on a Cisco IOS router.
Why this is correct
On Cisco IOS, the interface or global command 'no mpls ip propagate-ttl' stops the provider edge from copying IP TTL values into the MPLS header, so labelled packets traverse the core with a fixed TTL.
- ✗
The command 'no mpls ip ttl-propagate' is used to disable TTL propagation.
Why it's wrong here
The syntax is inverted: disabling propagation uses 'no mpls ip propagate-ttl' on the interface, not 'no mpls ip ttl-propagate'. It is tempting because the option names the right feature and intent, and would be correct if the command keywords matched the actual IOS configuration syntax.
- ✗
When TTL propagation is disabled, traceroute shows every LSR hop in the MPLS core.
Why it's wrong here
With propagation disabled, core LSRs are hidden from traceroute because the IP TTL is not copied from the MPLS header, so only ingress and egress LSRs appear. It is tempting because showing every LSR hop is precisely what traceroute does when TTL propagation is enabled.
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When TTL propagation is disabled, the TTL value is decremented normally at each LSR hop.
Why it's wrong here
Disabling TTL propagation makes the ingress LSR set the MPLS header TTL to 255 and stop copying it to the IP header, so core LSRs never decrement the visible IP TTL. It is tempting because per-hop decrement is exactly what happens when propagation is enabled.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.