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.
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.
In MPLS, what is the purpose of the TTL propagation feature?
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: 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.
✗
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
Static NAT maps one inside address to one outside address.
✗
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: 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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
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.
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: 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?
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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