- A
Disable MPLS on PE-CE links
Disabling MPLS on PE-CE links prevents customers from sending labeled packets.
- B
Enable MPLS TTL propagation to detect spoofing
Why wrong: TTL propagation does not prevent spoofing; it exposes the label stack.
- C
Enable MPLS on PE-CE links with TTL propagation disabled
Why wrong: Enabling MPLS on PE-CE links exposes the label stack and allows spoofing.
- D
Use explicit null labels on PE-CE links
Why wrong: Explicit null is for penultimate hop popping, not for security.
Question 52 of 1,000
350-501 Security and Services Practice Question
This 350-501 practice question tests your understanding of security and services. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
An MPLS L3VPN service provider wants to prevent label spoofing attacks where a customer could inject MPLS labels to bypass ACLs. Which configuration practice should be implemented on PE-CE links?
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
✓
Disable MPLS on PE-CE links
To prevent label spoofing, MPLS should be disabled on PE-CE links so that customers cannot send labeled packets. Additionally, disabling TTL propagation prevents label stack visibility.
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.
- ✓
Disable MPLS on PE-CE links
Why this is correct
Disabling MPLS on PE-CE links prevents customers from sending labeled packets.
Related concept
Standard ACLs match source addresses.
- ✗
Enable MPLS TTL propagation to detect spoofing
Why it's wrong here
TTL propagation does not prevent spoofing; it exposes the label stack.
- ✗
Enable MPLS on PE-CE links with TTL propagation disabled
Why it's wrong here
Enabling MPLS on PE-CE links exposes the label stack and allows spoofing.
- ✗
Use explicit null labels on PE-CE links
Why it's wrong here
Explicit null is for penultimate hop popping, not for security.
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: Disable MPLS on PE-CE links — To prevent label spoofing, MPLS should be disabled on PE-CE links so that customers cannot send labeled packets. Additionally, disabling TTL propagation prevents label stack visibility.
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.
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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