The correct answer is that the ACL blocks all traffic except established TCP and UDP. This is because the `established` keyword in a Cisco ACL matches only TCP segments with the ACK or RST bit set, meaning it permits return traffic from existing sessions while dropping initial SYN packets that start new connections. In the output, line 10 permits only established TCP traffic, line 20 permits all UDP, and line 30 denies everything else, so non-established TCP (like a new web request) is silently dropped. On the CCNP ENARSI 300-410 exam, this tests your understanding of stateful filtering behavior without a stateful firewall—a common trap is assuming `established` allows all TCP, when it actually blocks the three-way handshake’s first SYN. A quick memory tip: “ACK or RST means the session’s already been blessed.”
300-410 IPv4 Access Control Lists Practice Question
This 300-410 practice question tests your understanding of ipv4 access control lists. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 on Router R1:
R1# show ip access-lists
Extended IP access list 160
10 permit tcp any any established (200 matches)
20 permit udp any any (100 matches)
30 deny ip any any (50 matches)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The ACL blocks all traffic except established TCP and UDP.
The ACL 160 permits only established TCP sessions (using the 'established' keyword, which matches TCP segments with the ACK or RST bit set) and all UDP traffic, while denying everything else. This means non-established TCP traffic (e.g., initial SYN packets) is blocked, so the ACL effectively blocks all traffic except established TCP and UDP, making option B correct.
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 allows all UDP traffic.
Why it's wrong here
Line 20 permits UDP, but line 30 denies all other IP traffic, so UDP is permitted.
✓
The ACL blocks all traffic except established TCP and UDP.
Why this is correct
Line 30 denies all other IP traffic, which may block new TCP connections and other protocols.
Related concept
Read the scenario before looking for a memorised answer.
✗
The ACL permits all TCP traffic.
Why it's wrong here
Only established TCP is permitted; new TCP connections would be denied by line 30.
✗
The ACL is correctly configured for a firewall.
Why it's wrong here
Blocking all non-established TCP and non-UDP traffic may be too restrictive.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that an ACL with 'permit tcp any any established' allows all TCP traffic, when in fact it only permits TCP segments with the ACK or RST bit set, blocking initial SYN packets.
Detailed technical explanation
How to think about this question
The 'established' keyword in Cisco IOS ACLs matches TCP packets that have the ACK or RST bit set, allowing return traffic for outbound-initiated sessions but blocking new inbound TCP connections. This is a common stateful filtering technique, but it does not inspect UDP or ICMP, so UDP traffic is permitted entirely, which can be exploited. In real-world scenarios, this ACL would be used on an external interface to allow responses to internal requests while blocking unsolicited inbound TCP, but the open UDP policy is a significant vulnerability.
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: The ACL blocks all traffic except established TCP and UDP. — The ACL 160 permits only established TCP sessions (using the 'established' keyword, which matches TCP segments with the ACK or RST bit set) and all UDP traffic, while denying everything else. This means non-established TCP traffic (e.g., initial SYN packets) is blocked, so the ACL effectively blocks all traffic except established TCP and UDP, making option B correct.
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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