300-410 IPv4 Access Control Lists Practice Question
This 300-410 practice question tests your understanding of ipv4 access control lists. 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.
Examine the following configuration snippet:
```
interface GigabitEthernet0/0
ip access-group BLOCK_TELNET out
!
ip access-list extended BLOCK_TELNET
deny tcp any any eq 23
permit ip any any
```
What is the effect of applying this ACL outbound on GigabitEthernet0/0?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
It blocks Telnet traffic leaving the interface, but permits all other IP traffic.
The ACL is applied outbound on GigabitEthernet0/0, meaning it filters traffic as it leaves the interface. The first rule denies TCP traffic destined for port 23 (Telnet), and the second rule permits all other IP traffic. Therefore, Telnet sessions initiated from inside the network and exiting this interface are blocked, while all other traffic is allowed.
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.
✗
It blocks Telnet traffic entering the interface from the network.
Why it's wrong here
Incorrect. The ACL is applied outbound, so it filters traffic leaving the interface, not entering.
✓
It blocks Telnet traffic leaving the interface, but permits all other IP traffic.
Why this is correct
Correct. The outbound ACL denies Telnet and permits all other IP traffic.
Related concept
Read the scenario before looking for a memorised answer.
✗
It permits Telnet traffic and blocks all other IP traffic.
Why it's wrong here
Incorrect. The ACL denies Telnet and permits all other IP traffic.
✗
It has no effect because the ACL is missing a sequence number.
Why it's wrong here
Incorrect. Named ACLs do not require sequence numbers; the configuration is valid.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between inbound and outbound ACL application, and the trap here is that candidates mistakenly think an outbound ACL filters traffic entering the interface rather than leaving it.
Detailed technical explanation
How to think about this question
Outbound ACLs evaluate packets after the routing decision is made, just before the packet is forwarded out the interface. The 'deny tcp any any eq 23' entry matches any TCP segment with a destination port of 23, regardless of source or destination IP. In a real-world scenario, this would block internal users from reaching external Telnet servers but would not affect inbound Telnet traffic (e.g., to a server inside the network).
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 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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
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: It blocks Telnet traffic leaving the interface, but permits all other IP traffic. — The ACL is applied outbound on GigabitEthernet0/0, meaning it filters traffic as it leaves the interface. The first rule denies TCP traffic destined for port 23 (Telnet), and the second rule permits all other IP traffic. Therefore, Telnet sessions initiated from inside the network and exiting this interface are blocked, while all other traffic is allowed.
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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Question Discussion
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