- A
The ACL permits UDP port 22, which is not used by SSH; this could allow unwanted UDP traffic.
Correct. SSH uses TCP, so the UDP permit is unnecessary and potentially risky.
- B
The ACL is missing a deny statement at the end to block other traffic.
Why wrong: Incorrect. There is an implicit deny at the end of all ACLs; it is not required to be explicit.
- C
The ACL should use the 'established' keyword to allow return traffic.
Why wrong: Incorrect. The 'established' keyword is used for TCP sessions, but the ACL permits inbound SSH to a specific host, which is correct for inbound connections.
- D
The ACL should be applied outbound instead of inbound.
Why wrong: Incorrect. The direction depends on the desired filtering; inbound is appropriate for filtering traffic coming into the interface.
Quick Answer
The answer is that the ACL permits UDP port 22, which is a misconfiguration because SSH exclusively uses TCP port 22. The issue arises because the second permit statement allows any UDP traffic destined for port 22, even though SSH has no UDP component; this opens the door for unwanted or malicious UDP-based traffic to reach the host, potentially bypassing the intended security filter. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of application-layer protocols and ACL filtering logic, often appearing as a common trap where candidates overlook that SSH is strictly TCP-based. The key is to remember that while port 22 is synonymous with SSH, the protocol itself requires TCP, so permitting UDP on the same port creates a security gap. Memory tip: "SSH is TCP only—UDP on 22 is a security hole, not a feature."
300-410 IPv4 Access Control Lists Practice Question
This 300-410 practice question tests your understanding of ipv4 access control lists. 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.
Given the following configuration: ```
interface GigabitEthernet0/2 ip access-group FILTER_SSH in
!
ip access-list extended FILTER_SSH permit tcp any host 192.168.1.10 eq 22 permit udp any host 192.168.1.10 eq 22
``` What is the issue with this ACL?
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
The ACL permits UDP port 22, which is not used by SSH; this could allow unwanted UDP traffic.
The ACL permits both TCP and UDP traffic to destination port 22. SSH uses only TCP port 22; UDP port 22 is not used by SSH. The second permit statement allows any UDP traffic to port 22, which could be unwanted (e.g., UDP-based attacks or non-SSH services). This is the issue.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The ACL permits UDP port 22, which is not used by SSH; this could allow unwanted UDP traffic.
- ✗
The ACL is missing a deny statement at the end to block other traffic.
Why it's wrong here
Incorrect. There is an implicit deny at the end of all ACLs; it is not required to be explicit.
- ✗
The ACL should use the 'established' keyword to allow return traffic.
- ✗
The ACL should be applied outbound instead of inbound.
Why it's wrong here
Incorrect. The direction depends on the desired filtering; inbound is appropriate for filtering traffic coming into the interface.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that SSH uses both TCP and UDP, or that permitting UDP port 22 is harmless, when in fact only TCP port 22 is valid for SSH.
Trap categories for this question
Keyword trap
Incorrect. The 'established' keyword is used for TCP sessions, but the ACL permits inbound SSH to a specific host, which is correct for inbound connections.
Detailed technical explanation
How to think about this question
SSH (Secure Shell) is defined in RFC 4251 and operates over TCP port 22. UDP port 22 is not assigned to any standard service; permitting it could allow malicious or unwanted UDP traffic to reach the host. In real-world scenarios, this misconfiguration might be exploited by attackers sending UDP floods to port 22, potentially bypassing security controls that expect only TCP. Cisco ACLs process entries sequentially and have an implicit deny at the end, so the UDP permit is a security gap.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Questions learners often ask
What does this 300-410 question test?
IPv4 Access Control Lists — This question tests IPv4 Access Control Lists — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The ACL permits UDP port 22, which is not used by SSH; this could allow unwanted UDP traffic. — The ACL permits both TCP and UDP traffic to destination port 22. SSH uses only TCP port 22; UDP port 22 is not used by SSH. The second permit statement allows any UDP traffic to port 22, which could be unwanted (e.g., UDP-based attacks or non-SSH services). This is the issue.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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