- A
QoS policy to rate-limit attack traffic
Why wrong: Rate-limiting may not fully mitigate high-volume attacks.
- B
An inbound ACL blocking the attack source IPs
Why wrong: ACLs are not scalable for large attacks and don't use BGP.
- C
A static route to Null0 for the victim IP and a BGP community to trigger blackholing
This is the standard S/RTBH mechanism.
- D
PBR to redirect traffic to a scrubbing center
Why wrong: That's IDMS, not S/RTBH.
Question 959 of 1,000
350-501 Security and Services Practice Question
This 350-501 practice question tests your understanding of security and services. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
During a DDoS attack, a service provider uses Cisco Peakflow to detect anomalous traffic and then triggers S/RTBH. What must be configured on the router to black hole attack traffic using a /32 null route?
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
✓
A static route to Null0 for the victim IP and a BGP community to trigger blackholing
S/RTBH relies on BGP to propagate a /32 route with a specific community (commonly no-export) pointing to a null interface (e.g., Null0). The trigger router sets the next-hop to a static route pointing to Null0 and advertises it via BGP.
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.
- ✗
QoS policy to rate-limit attack traffic
Why it's wrong here
Rate-limiting may not fully mitigate high-volume attacks.
- ✗
An inbound ACL blocking the attack source IPs
Why it's wrong here
ACLs are not scalable for large attacks and don't use BGP.
- ✓
A static route to Null0 for the victim IP and a BGP community to trigger blackholing
Why this is correct
This is the standard S/RTBH mechanism.
Related concept
Standard ACLs match source addresses.
- ✗
PBR to redirect traffic to a scrubbing center
Why it's wrong here
That's IDMS, not S/RTBH.
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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Security and Services — study guide chapter
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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: A static route to Null0 for the victim IP and a BGP community to trigger blackholing — S/RTBH relies on BGP to propagate a /32 route with a specific community (commonly no-export) pointing to a null interface (e.g., Null0). The trigger router sets the next-hop to a static route pointing to Null0 and advertises it via BGP.
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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