Details
This command places the router in access-list configuration mode, where you must define the denied or permitted access conditions by using the deny and permit commands.
Solution
Apply the access-group for the external (untrusted) interface
hostname(config)#interface {external_interface}
hostname(config-if)#ip access-group {name | number} in
Supportive Information
The following resource is also helpful.
This security hardening control applies to the following category of controls within NIST 800-53: System and Communications Protection.This control applies to the following type of system Cisco.