- A
The 'established' keyword matches TCP packets that have the ACK or RST bit set.
Correct. The 'established' keyword matches TCP packets that have the ACK or RST bit set, allowing return traffic for established sessions.
- B
The 'established' keyword can be applied to extended ACLs for UDP traffic to permit return packets.
Why wrong: Incorrect. The 'established' keyword is specific to TCP; it cannot be applied to UDP traffic because UDP has no flags.
- C
The 'established' keyword checks the state of the session in the router's state table.
Why wrong: Incorrect. The 'established' keyword is stateless; it does not check the router's state table but rather inspects the TCP flags in the packet header.
- D
The 'established' keyword can be used in both standard and extended ACLs.
Correct. The 'established' keyword can be used in both standard and extended ACLs, though it is most commonly associated with extended ACLs.
- E
The 'established' keyword will match a TCP SYN packet sent from a client to initiate a connection.
Why wrong: Incorrect. The 'established' keyword will not match a TCP SYN packet because a SYN packet typically has only the SYN flag set, not ACK or RST.
established Keyword in IPv4 ACLs
This 300-410 practice question tests your understanding of ipv4 access control lists. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
Which TWO statements about the behavior of IPv4 ACLs with the 'established' keyword are true? (Choose TWO.)
Quick Answer
The answer is that the 'established' keyword in IPv4 ACLs matches TCP packets with the ACK or RST bit set. This is correct because the keyword is designed to permit return traffic from an existing TCP session while blocking initial connection attempts; a SYN-only packet, which starts a new session, will not have these bits set, so it is denied. On the Cisco CCNP ENARSI 300-410 exam, this concept tests your understanding of stateful-like filtering with stateless ACLs, often appearing in questions about securing network traffic or troubleshooting asymmetric routing. A common trap is assuming the keyword matches all TCP packets or that it works with UDP, but it strictly inspects the TCP control bits. For a quick memory tip, think "ACK or RST means established, SYN alone is denied."
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 'established' keyword matches TCP packets that have the ACK or RST bit set.
Options A and D are true statements about the 'established' keyword. Option A correctly states that it matches TCP packets with the ACK or RST bit set. Option D correctly states that it can be used in both standard and extended ACLs; although standard ACLs are typically limited to source IP filtering, the 'established' keyword can be applied in standard ACLs to filter based on TCP flags.
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 'established' keyword matches TCP packets that have the ACK or RST bit set.
Why this is correct
Correct. The 'established' keyword matches TCP packets that have the ACK or RST bit set, allowing return traffic for established sessions.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The 'established' keyword can be applied to extended ACLs for UDP traffic to permit return packets.
- ✗
The 'established' keyword checks the state of the session in the router's state table.
Why it's wrong here
Incorrect. The 'established' keyword is stateless; it does not check the router's state table but rather inspects the TCP flags in the packet header.
- ✓
The 'established' keyword can be used in both standard and extended ACLs.
Why this is correct
Correct. The 'established' keyword can be used in both standard and extended ACLs, though it is most commonly associated with extended ACLs.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The 'established' keyword will match a TCP SYN packet sent from a client to initiate a connection.
Why it's wrong here
Incorrect. The 'established' keyword will not match a TCP SYN packet because a SYN packet typically has only the SYN flag set, not ACK or RST.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that the 'established' keyword performs stateful inspection or works with UDP, when in reality it is a simple stateless TCP flag check limited to extended ACLs.
Trap categories for this question
Keyword trap
Incorrect. The 'established' keyword is specific to TCP; it cannot be applied to UDP traffic because UDP has no flags.
Detailed technical explanation
How to think about this question
The 'established' keyword works by examining the TCP header's control flags; specifically, it matches packets where the ACK or RST bit is set to 1. This is a stateless mechanism—it does not track sessions but relies on the fact that return packets in a TCP connection typically have the ACK bit set. In real-world scenarios, this can be used to permit responses from a web server while blocking unsolicited inbound SYN packets, but it is not foolproof against spoofed ACK packets.
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.
Visual reference
What to study next
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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 'established' keyword matches TCP packets that have the ACK or RST bit set. — Options A and D are true statements about the 'established' keyword. Option A correctly states that it matches TCP packets with the ACK or RST bit set. Option D correctly states that it can be used in both standard and extended ACLs; although standard ACLs are typically limited to source IP filtering, the 'established' keyword can be applied in standard ACLs to filter based on TCP flags.
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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