On a Cisco router, which command allows you to verify whether an IP access list has been applied to a particular interface and in which direction it is active?

Introduction / Context:
After you create an access control list on a Cisco router, you must apply it to an interface in either the inbound or outbound direction for it to take effect. Troubleshooting connectivity often requires you to confirm whether a particular access list is actually active on an interface and which direction it is filtering. Cisco IOS offers several show commands that display ACL information, and you must know which one gives you interface level attachment details.

Given Data / Assumptions:

An IP access list has been defined on the router configuration.
You want to know if this access list is applied to a specific interface.
You also want to see whether the ACL is applied inbound, outbound or not at all.
You are working with standard Cisco IOS commands.

Concept / Approach:
The show access-lists command displays the contents of access lists and any hit counters but does not tell you which interfaces they are bound to. The show running-config command reveals configuration lines but can be long and harder to parse quickly. The show ip interface command, when run for a specific interface or for all interfaces, includes a section that explicitly states whether an inbound or outbound access list is set. Therefore show ip interface is the best tool to answer the question of whether an ACL is enabled on a particular interface.

Step-by-Step Solution:
Consider the requirements: you want to confirm that an ACL is enabled on a particular interface and see the direction of application. The show access-lists command will show the rules in each list and hit counts, but it does not list bound interfaces. The show ip interface command provides detailed status for each interface, including lines like Inbound access list is 101 or Outbound access list is not set. By running show ip interface or show ip interface serial0 or show ip interface fastethernet0/0, you can see exactly which ACL, if any, is attached to that interface. Therefore, among the options, show ip interface is the correct command for determining whether an IP access list is enabled on a particular interface.

Verification / Alternative check:
In lab environments, you can configure an ACL and apply it with ip access-group 101 in under an interface, then use show ip interface to check the result. The output will clearly mention the list number for inbound and outbound directions. This direct evidence confirms that show ip interface is the intended verification command rather than show access-lists.

Why Other Options Are Wrong:
Option a, show ip port, is not a standard Cisco IOS command and does not relate to ACLs.
Option b, show access-lists, displays the definitions of ACLs and counters but not where they are applied.
Option d, show access-lists interface, is not a valid IOS show command in classic router syntax.
Option e, show running-config access-list, is also not a standard command; you would instead use show running-config and visually search, but that is not the concise interface focused answer expected.

Common Pitfalls:
A typical mistake is to rely only on show access-lists and assume that if a list exists it must be in use. This can lead to confusion when traffic does not pass as expected. Always remember that ACLs are inactive until applied to an interface or vty line. Learning to quickly read show ip interface output saves a lot of troubleshooting time.

Final Answer:
The correct command is show ip interface, which displays whether an IP access list is enabled on a specific interface and in which direction.