- A
Disabling TTL propagation on PE-CE interfaces
Disabling TTL propagation hides the MPLS hops from the customer.
- B
Enabling MPLS on all interfaces including CE-facing
Why wrong: MPLS should be disabled on CE-facing interfaces to prevent label spoofing.
- C
Enabling TTL propagation on all PE routers
Why wrong: TTL propagation exposes the MPLS network to customers.
- D
Using IP explicit null labels on all LSPs
Why wrong: IP explicit null is for QoS, not anti-spoofing.
Question 402 of 1,000
350-501 Security and Services Practice Question
This 350-501 practice question tests your understanding of security and services. 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.
Which MPLS security best practice helps prevent label spoofing attacks where an attacker injects MPLS packets with a forged label stack to bypass ACLs?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"best"Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
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 on PE-CE interfaces
Disabling MPLS TTL propagation prevents an attacker from gleaning information about the MPLS network. More importantly, ensuring that the label stack is not exposed to customers and that ingress filtering is applied. Among options, disabling TTL propagation is a direct measure against label spoofing by hiding the network topology.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
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 on PE-CE interfaces
Why this is correct
Disabling TTL propagation hides the MPLS hops from the customer.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
- ✗
Enabling MPLS on all interfaces including CE-facing
Why it's wrong here
MPLS should be disabled on CE-facing interfaces to prevent label spoofing.
- ✗
Enabling TTL propagation on all PE routers
Why it's wrong here
TTL propagation exposes the MPLS network to customers.
- ✗
Using IP explicit null labels on all LSPs
Why it's wrong here
IP explicit null is for QoS, not anti-spoofing.
Common exam traps
Common exam trap: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
- Standard ACLs match source addresses.
- Extended ACLs can match source, destination, protocol and ports.
- The first matching ACL entry is used.
- There is usually an implicit deny at the end.
TExam Day Tips
- Check inbound versus outbound direction.
- Read the ACL from top to bottom.
- Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Real-world example
How this comes up in practice
A security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
What to study next
Got this wrong? Here's your next step.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related 350-501 ACL questions on filtering logic and placement.
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FAQ
Questions learners often ask
What does this 350-501 question test?
Security and Services — This question tests Security and Services — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: Disabling TTL propagation on PE-CE interfaces — Disabling MPLS TTL propagation prevents an attacker from gleaning information about the MPLS network. More importantly, ensuring that the label stack is not exposed to customers and that ingress filtering is applied. Among options, disabling TTL propagation is a direct measure against label spoofing by hiding the network topology.
What should I do if I get this 350-501 question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related 350-501 ACL questions on filtering logic and placement.
Are there clue words in this question I should notice?
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Standard ACLs match source addresses.
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Last reviewed: Jul 4, 2026
This 350-501 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 350-501 exam.
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