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.
A network engineer runs the following command to troubleshoot an IPv4 Access Control Lists issue:
R1# debug ip packet 110
IP packet debugging is on for access list 110
*Mar 1 00:15:22.345: IP: s=10.1.1.1 (GigabitEthernet0/0), d=10.2.2.2, len 100, proto TCP, flags 0x2, sport 12345, dport 23, access list 110: matched line 10 deny tcp host 10.1.1.1 host 10.2.2.2 eq 23
*Mar 1 00:15:22.346: IP: s=10.1.1.1 (GigabitEthernet0/0), d=10.2.2.2, len 100, proto TCP, flags 0x10, sport 12345, dport 23, access list 110: matched line
10 deny tcp host 10.1.1.1 host 10.2.2.2 eq 23
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Telnet traffic from 10.1.1.1 to 10.2.2.2 is being denied by ACL 110.
The debug output shows packets with source IP 10.1.1.1 and destination IP 10.2.2.2, protocol TCP, destination port 23 (Telnet), and the log explicitly states 'matched line 10 deny tcp host 10.1.1.1 host 10.2.2.2 eq 23'. This confirms that ACL 110 is denying Telnet traffic from 10.1.1.1 to 10.2.2.2. The flags 0x2 (SYN) and 0x10 (ACK) indicate the initial and subsequent packets of the Telnet session are both being denied.
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.
✓
Telnet traffic from 10.1.1.1 to 10.2.2.2 is being denied by ACL 110.
Why this is correct
The debug shows the packets match the deny line.
Related concept
Read the scenario before looking for a memorised answer.
✗
Telnet traffic from 10.1.1.1 to 10.2.2.2 is being permitted by ACL 110.
Why it's wrong here
The match is on a deny line.
✗
ACL 110 is applied outbound on GigabitEthernet0/0.
Why it's wrong here
The debug does not show direction; it only shows the interface where the packet was seen.
✗
ACL 110 has no line 10.
Why it's wrong here
The debug shows line 10.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may misinterpret the 'matched line 10 deny' as a permit action or assume the ACL is applied outbound based on the source interface, but the debug only shows the packet's ingress interface and the ACL match result, not the ACL's application direction.
Trap categories for this question
Command / output trap
The debug does not show direction; it only shows the interface where the packet was seen.
Detailed technical explanation
How to think about this question
The 'debug ip packet' command with an ACL number filters debugging to only packets matching that ACL. The flags field (e.g., 0x2 for SYN, 0x10 for ACK) reveals TCP handshake states; here, both SYN and ACK packets are denied, meaning the entire session is blocked. In real-world troubleshooting, this output helps identify if an ACL is incorrectly denying traffic, such as blocking Telnet (port 23) when it should be permitted for management access.
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.
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: Telnet traffic from 10.1.1.1 to 10.2.2.2 is being denied by ACL 110. — The debug output shows packets with source IP 10.1.1.1 and destination IP 10.2.2.2, protocol TCP, destination port 23 (Telnet), and the log explicitly states 'matched line 10 deny tcp host 10.1.1.1 host 10.2.2.2 eq 23'. This confirms that ACL 110 is denying Telnet traffic from 10.1.1.1 to 10.2.2.2. The flags 0x2 (SYN) and 0x10 (ACK) indicate the initial and subsequent packets of the Telnet session are both being denied.
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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