Which ACL entry permits TCP packets that include the ACK flag?
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Correct answer: access-list 110 permit tcp any any eq 21 established.
Why this is the answer
The established keyword in an extended ACL permits TCP packets that have the ACK or RST bits set. This is a common method to allow return traffic for established connections while still filtering new incoming connections. The correct answer specifies TCP, uses a valid extended ACL number (110), and the established keyword. The option access-list 110 permit tcp any any eq 21 tcp-ack uses tcp-ack, which is not a valid keyword in Cisco IOS ACLs. The option access-list 10 permit tcp any any eq 21 established uses a standard ACL number (10), but established is only valid for extended ACLs. The option access-list 10 permit ip any any eq 21 tcp-ack uses a standard ACL number (10), specifies ip instead of tcp, and uses the invalid tcp-ack keyword.
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