The answer is to remove the 'established' keyword from the ACL to allow initial SYN packets. The 'established' keyword is a common ACL feature that only permits TCP segments with the ACK or RST bit set, meaning it allows return traffic from an existing connection but blocks the very first SYN packet needed to start a new TCP session. In this scenario, users can reach the server briefly because a prior outbound flow or a temporary permit allows the initial connection, but once that state expires, the ACL drops all new SYN packets from the 192.168.10.0/24 subnet, causing connectivity to fail until the interface is cleared. On the CCNA 200-301 v2 exam, this tests your understanding of how ACLs filter TCP traffic based on flags, and it’s a classic trap where candidates assume 'established' secures all traffic when it actually blocks new sessions. Remember the memory tip: "Established means ACK, not SYN—if you want new connections, let the first handshake in."
CCNA Network Services and Security Practice Question
This 200-301 practice question tests your understanding of network services and security. 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.
Exhibit
Router# show running-config | section interface GigabitEthernet0/1
interface GigabitEthernet0/1
ip address 192.168.10.1 255.255.255.0
ip access-group OUTBOUND_FILTER out
duplex auto
speed auto
!
Router# show running-config | section ip access-list extended OUTBOUND_FILTER
ip access-list extended OUTBOUND_FILTER
permit tcp 192.168.10.0 0.0.0.255 host 10.10.10.10 established
permit icmp 192.168.10.0 0.0.0.255 host 10.10.10.10 echo-request
deny ip any any
A network administrator is troubleshooting connectivity from the 192.168.10.0/24 subnet to the server at 10.10.10.10. Users report that they can reach the server initially, but after a few minutes, connectivity drops and only returns after the interface is cleared. The administrator reviews the router's running configuration and ACL configuration. What is the most likely cause of the intermittent connectivity loss?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Router# show running-config | section interface GigabitEthernet0/1
interface GigabitEthernet0/1
ip address 192.168.10.1 255.255.255.0
ip access-group OUTBOUND_FILTER out
duplex auto
speed auto
!
Router# show running-config | section ip access-list extended OUTBOUND_FILTER
ip access-list extended OUTBOUND_FILTER
permit tcp 192.168.10.0 0.0.0.255 host 10.10.10.10 established
permit icmp 192.168.10.0 0.0.0.255 host 10.10.10.10 echo-request
deny ip any any
A
Remove the 'established' keyword from the ACL to allow initial TCP connections.
The 'established' keyword only permits TCP packets with the ACK or RST bit set, which are responses to established sessions. The initial SYN packet does not have these bits, so it is denied. Removing 'established' allows the TCP three-way handshake to complete.
B
Apply the ACL inbound on the interface instead of outbound.
Why wrong: Changing the direction would affect traffic differently, but the core issue is the 'established' keyword blocking initial SYN packets. Inbound application would not solve the problem; it would simply filter traffic before routing.
C
Add a permit statement for UDP traffic to allow DNS resolution.
Why wrong: The issue is with TCP and ICMP traffic. UDP is not mentioned in the symptom, and adding a permit for UDP would not fix the TCP connection failure.
D
Change the wildcard mask to 0.0.0.255 to match the subnet more accurately.
Why wrong: The wildcard mask 0.0.0.255 is already correct for matching the 192.168.10.0/24 subnet. Changing it would either broaden or narrow the match, but it would not resolve the 'established' keyword issue.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Remove the 'established' keyword from the ACL to allow initial TCP connections.
The 'established' keyword in an ACL permits TCP packets that have the ACK or RST bit set, which are responses to outbound connections. However, it does not permit the initial SYN packet from the client to the server. Since users can reach the server initially (likely due to a temporary permit or stateful inspection elsewhere), but connectivity drops after a few minutes, the issue is that the ACL is blocking the initial SYN packets needed to establish new TCP connections. Removing the 'established' keyword (or adding a permit for the initial SYN) resolves this by allowing all TCP traffic from the source subnet.
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.
✓
Remove the 'established' keyword from the ACL to allow initial TCP connections.
Why this is correct
The 'established' keyword only permits TCP packets with the ACK or RST bit set, which are responses to established sessions. The initial SYN packet does not have these bits, so it is denied. Removing 'established' allows the TCP three-way handshake to complete.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Apply the ACL inbound on the interface instead of outbound.
Why it's wrong here
Changing the direction would affect traffic differently, but the core issue is the 'established' keyword blocking initial SYN packets. Inbound application would not solve the problem; it would simply filter traffic before routing.
✗
Add a permit statement for UDP traffic to allow DNS resolution.
Why it's wrong here
The issue is with TCP and ICMP traffic. UDP is not mentioned in the symptom, and adding a permit for UDP would not fix the TCP connection failure.
✗
Change the wildcard mask to 0.0.0.255 to match the subnet more accurately.
Why it's wrong here
The wildcard mask 0.0.0.255 is already correct for matching the 192.168.10.0/24 subnet. Changing it would either broaden or narrow the match, but it would not resolve the 'established' keyword issue.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓Remove the 'established' keyword from the ACL to allow initial TCP connections.Correct answer▾
Why this is correct
The 'established' keyword only permits TCP packets with the ACK or RST bit set, which are responses to established sessions. The initial SYN packet does not have these bits, so it is denied. Removing 'established' allows the TCP three-way handshake to complete.
✗Apply the ACL inbound on the interface instead of outbound.Wrong answer — click to see why▾
Why this is wrong here
The direction does not change the fact that the 'established' keyword only permits packets with ACK/RST flags, which excludes the initial SYN.
✗Add a permit statement for UDP traffic to allow DNS resolution.Wrong answer — click to see why▾
Why this is wrong here
The problem is specifically with TCP connections and ICMP echo-replies, not DNS or UDP.
✗Change the wildcard mask to 0.0.0.255 to match the subnet more accurately.Wrong answer — click to see why▾
Why this is wrong here
The wildcard mask is already appropriate; the problem lies in the 'established' keyword.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that the 'established' keyword permits all TCP traffic, when in fact it only permits packets with the ACK or RST bit set, blocking the initial SYN packet needed to start a new connection.
Trap categories for this question
Keyword trap
Changing the direction would affect traffic differently, but the core issue is the 'established' keyword blocking initial SYN packets. Inbound application would not solve the problem; it would simply filter traffic before routing.
Detailed technical explanation
How to think about this question
The 'established' keyword matches TCP segments with the ACK or RST bits set, which are typical of established connections, but it does not match the initial SYN segment. In a stateless ACL, this can cause asymmetric traffic filtering where return traffic is permitted but new connection attempts are dropped. A real-world scenario is when a router uses an ACL with 'established' to allow outbound-initiated traffic while blocking inbound connections, but fails to permit the initial SYN from the client, leading to timeouts after the first connection's state expires.
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 200-301 question in full detail.
Network Services and Security — This question tests Network Services and Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Remove the 'established' keyword from the ACL to allow initial TCP connections. — The 'established' keyword in an ACL permits TCP packets that have the ACK or RST bit set, which are responses to outbound connections. However, it does not permit the initial SYN packet from the client to the server. Since users can reach the server initially (likely due to a temporary permit or stateful inspection elsewhere), but connectivity drops after a few minutes, the issue is that the ACL is blocking the initial SYN packets needed to establish new TCP connections. Removing the 'established' keyword (or adding a permit for the initial SYN) resolves this by allowing all TCP traffic from the source subnet.
What should I do if I get this 200-301 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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