Quick Answer
The answer is that the command is rejected by the IOS parser. This occurs because in an extended IPv4 ACL, the protocol keyword is mandatory; the parser cannot infer which protocol to filter when only a source and destination are provided, so it returns an error. The correct syntax requires a protocol—such as ip, tcp, or udp—immediately after the permit or deny keyword, and omitting it violates the ACL structure. On the Cisco CCNP ENARSI 300-410 exam, this tests your understanding of extended ACL syntax fundamentals, often appearing as a trap where candidates assume a default protocol like ip is applied. A common memory tip is to remember the extended ACL mantra: “protocol first, then source, then destination,” and that omitting the protocol keyword is a syntax error, not a silent default.
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.
In an extended IPv4 ACL, what is the default action if only a source and destination are specified without a protocol?
The ACL matches all IP traffic.
Why wrong: Without a protocol, the command is invalid.
The ACL matches only TCP traffic.
Why wrong: No default protocol is assumed.
The command is rejected by the IOS parser.
Extended ACL syntax requires a protocol field; omission causes a syntax error.
The ACL matches only UDP traffic.
Why wrong: No default protocol; the command fails.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The command is rejected by the IOS parser.
In an extended IPv4 ACL, the protocol keyword is mandatory. If you omit it, the IOS parser rejects the command because it cannot determine which protocol to filter. The correct syntax requires a protocol (e.g., ip, tcp, udp) after the permit or deny keyword; without it, the parser returns an error.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
The ACL matches all IP traffic.
Why it's wrong here
Without a protocol, the command is invalid.
The ACL matches only TCP traffic.
Why it's wrong here
No default protocol is assumed.
The command is rejected by the IOS parser.
Why this is correct
Extended ACL syntax requires a protocol field; omission causes a syntax error.
Related concept
Read the scenario before looking for a memorised answer.
The ACL matches only UDP traffic.
Why it's wrong here
No default protocol; the command fails.
Common exam traps
Cisco often tests the mandatory nature of the protocol field in extended ACLs, trapping candidates who assume a default protocol (like IP, TCP, or UDP) is applied when none is specified.
Trap categories for this question
Command / output trap
Without a protocol, the command is invalid.
Detailed technical explanation
Under the hood, the Cisco IOS parser for extended ACLs (e.g., access-list 100 permit ...) expects a protocol number or name (0-255) immediately after the permit/deny keyword. If omitted, the parser fails with '% Incomplete command' or similar error. This is defined in Cisco's ACL configuration guidelines; the 'ip' keyword is used to match all IP traffic, but it must be explicitly stated. In real-world scenarios, forgetting the protocol is a common syntax error that prevents ACL application, leading to no filtering until corrected.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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.
What to study next
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FAQ
IPv4 Access Control Lists — This question tests IPv4 Access Control Lists — Read the scenario before looking for a memorised answer..
The correct answer is: The command is rejected by the IOS parser. — In an extended IPv4 ACL, the protocol keyword is mandatory. If you omit it, the IOS parser rejects the command because it cannot determine which protocol to filter. The correct syntax requires a protocol (e.g., ip, tcp, udp) after the permit or deny keyword; without it, the parser returns an error.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.
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