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300-410 Practice Question: In an extended IPv4 ACL, what is the default…
In an extended IPv4 ACL, what is the default action if only a source and destination are specified without a protocol?
⚠ Common exam trap
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.
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 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.
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 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.
- ✗
The ACL matches only UDP traffic.
Why it's wrong here
No default protocol; the command fails.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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