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Zorluk: KolayAccess Control Lists (Standard and Extended IPv4 ACLs)

A network technician configures a standard IPv4 access control list on a Cisco router using the command `access-list 5 permit host 172.16.10.15` and applies it inbound on an interface. What happens when host 172.16.10.20 sends traffic into that interface?

  1. The traffic is dropped because of the implicit deny clause at the end of the access control list.Cevap
  2. B
    The traffic is permitted because it originates from the same IP subnet as the permitted host.
  3. C
    The traffic is permitted by default until an explicit deny statement is added to the configuration.
  4. D
    The traffic is forwarded because standard access lists only drop traffic when configured with specific deny statements.

Cevap

The traffic is dropped because of the implicit deny clause at the end of the access control list.
Every Cisco IPv4 access control list automatically ends with an implicit deny statement (such as `deny any`). When host 172.16.10.20 sends packets into the interface, the router evaluates the packet against the explicit rule permitting host 172.16.10.15. Because the source address does not match, processing reaches the end of the list where the implicit deny drops the packet.

Adım Adım Çözüm

1
Inspect the explicit configuration line in the access control list.
The ACL contains only `access-list 5 permit host 172.16.10.15`.
Cisco IOS evaluates ACL entries sequentially against incoming packets.
2
Compare incoming packet source IP address 172.16.10.20 to the configured rule.
The IP address 172.16.10.20 does not match 172.16.10.15.
The 'host' keyword implies a wildcard mask of 0.0.0.0, matching only one exact address.
3
Evaluate the default behavior when no explicit rules match.
The packet falls through to the hidden implicit deny entry at the end of the ACL and is dropped.
Every IPv4 ACL in Cisco IOS automatically ends with an implicit `deny ip any` clause.

Anahtar Kavram

ACL Implicit Deny Clause
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