A network administrator configures a single line standard IPv4 Access Control List (ACL) on a Cisco IOS router as follows:
`access-list 10 permit 192.168.1.50 0.0.0.0`
This ACL is applied inbound on interface GigabitEthernet0/0. What happens to incoming IP traffic with a source IPv4 address of 192.168.1.20 when it reaches interface GigabitEthernet0/0?
- The traffic is dropped because of the invisible implicit deny any clause at the end of the ACL.Answer
- BThe traffic is permitted because standard ACLs allow all traffic by default unless explicitly denied.
- CThe traffic is forwarded because single-statement ACLs only filter the specified host and ignore all other hosts.
- DThe traffic bypasses interface processing because standard IPv4 ACLs can only be applied outbound.
Answer
The traffic is dropped because of the implicit deny any clause at the end of the access control list.
In Cisco IOS, every Access Control List ends with an unwritten, invisible implicit deny clause (`deny ip any any` or `deny any`). Because the packet from source address 192.168.1.20 does not match the explicit permit rule for host 192.168.1.50, it falls through to the end of the list and is dropped.
Step-by-Step Solution
Key Concept
ACL Implicit Deny Clause