[2024] CCNA Interview Questions on ACLs

Access Control Lists (ACLs) are a critical component of network security, especially in Cisco networks. ACLs help control the flow of traffic into and out of network interfaces, making them a fundamental topic in the CCNA certification exam. Understanding ACLs not only prepares you for the exam but also equips you with practical skills to manage and secure real-world networks. This article presents a comprehensive list of CCNA interview questions on ACLs, designed to help you gauge your understanding and readiness for any related interview.

1. What is an Access Control List (ACL) in networking?

An Access Control List (ACL) is a set of rules that control network traffic. It defines whether to permit or deny packets based on source and destination IP addresses, protocol types, and other packet attributes. ACLs are used to enhance security by filtering traffic in a network.

2. How many types of ACLs are there in Cisco devices, and what are they?

There are two primary types of ACLs in Cisco devices:

• Standard ACLs: These filter traffic based on the source IP address only.
• Extended ACLs: These filter traffic based on source and destination IP addresses, protocols, port numbers, and other parameters.

3. Where should you apply a standard ACL in a network?

Standard ACLs should be applied as close to the destination as possible because they filter traffic based only on the source IP address. Applying them too close to the source might unintentionally block legitimate traffic.

4. Where should you apply an extended ACL in a network?

Extended ACLs should be applied as close to the source as possible. This minimizes unnecessary traffic in the network by filtering out unwanted traffic before it traverses the network.

5. What is the difference between a numbered ACL and a named ACL?

• Numbered ACL: An ACL identified by a number (1-99 for standard, 100-199 for extended).
• Named ACL: An ACL identified by a descriptive name. Named ACLs provide more flexibility as they allow adding or removing specific rules without removing the entire ACL.

6. How do you configure a standard ACL in a Cisco router?

To configure a standard ACL, use the following commands:

This example permits traffic from the 192.168.1.0 network and applies the ACL to the incoming traffic on the g0/0 interface.

7. How do you configure an extended ACL in a Cisco router?

To configure an extended ACL, use the following commands:

This example permits HTTP traffic from the 192.168.1.0 network.

8. What is the implicit "deny all" rule in ACLs?

In Cisco ACLs, there is an implicit "deny all" rule at the end of every ACL. This means that if a packet does not match any of the permit rules in the ACL, it will be denied by default.

9. Can you modify a numbered ACL?

Modifying a numbered ACL is more complex than a named ACL because you typically need to delete the ACL and recreate it with the necessary changes. Named ACLs offer more flexibility in modification.

10. How does an ACL process traffic?

An ACL processes traffic in a sequential manner, from top to bottom. When a packet matches a rule, the ACL stops processing further rules. If the packet does not match any rule, the implicit "deny all" rule applies.

11. What are the best practices for writing ACLs?

Some best practices include:

• Always apply the most specific rules at the top of the ACL.
• Use "deny" rules before "permit" rules for unwanted traffic.
• Test ACLs in a lab environment before deploying them in production.

12. What is the difference between "inbound" and "outbound" ACLs?

• Inbound ACL: Filters traffic entering an interface before it is routed.
• Outbound ACL: Filters traffic after it has been routed, as it exits the interface.

13. How do ACLs enhance network security?

ACLs enhance network security by restricting unauthorized access, controlling the flow of traffic, and preventing specific types of traffic from entering or leaving a network. They can be used to prevent attacks, limit access to critical resources, and enforce network policies.

14. What is the impact of incorrectly configured ACLs?

Incorrectly configured ACLs can block legitimate traffic, leading to network outages or degraded performance. They can also fail to block malicious traffic, leaving the network vulnerable to attacks.

15. How do you troubleshoot ACL issues?

Troubleshooting ACL issues involves checking the ACL configuration, ensuring that the rules are correctly ordered, verifying the interfaces to which the ACL is applied, and using tools like packet tracers or debuggers to analyze the flow of traffic.

16. What is the purpose of a "wildcard mask" in ACLs, and how does it differ from a subnet mask?

A wildcard mask in ACLs is used to specify which bits of an IP address should be matched and which should be ignored. Unlike a subnet mask, where 1 bits indicate the network portion and 0 bits indicate the host portion, in a wildcard mask, 0 bits indicate that the corresponding bit in the IP address must match, and 1 bits indicate that the bit can be ignored.

17. How can you verify which ACLs are applied to an interface on a Cisco router?

To verify which ACLs are applied to an interface, you can use the following command:

This command displays whether an ACL is applied and whether it is inbound or outbound.

18. What is the significance of the "log" option in an ACL rule?

The "log" option in an ACL rule allows you to generate a log message every time the rule matches a packet. This is useful for monitoring and troubleshooting, as it provides visibility into which traffic is being permitted or denied by the ACL.

19. What happens if multiple ACLs are applied to a single interface?

Only one ACL can be applied per direction (inbound or outbound) on an interface. If you attempt to apply another ACL in the same direction, the existing one will be replaced.

20. Explain the difference between standard ACLs and extended ACLs in terms of their numbering ranges.

• Standard ACLs: Numbered from 1 to 99 and 1300 to 1999.
• Extended ACLs: Numbered from 100 to 199 and 2000 to 2699.

21. Can an ACL be used to filter traffic based on the type of service (ToS) or DSCP value?

Yes, extended ACLs can filter traffic based on the Type of Service (ToS) or Differentiated Services Code Point (DSCP) values, allowing for more granular traffic control based on the priority or service level.

22. What is the function of a "reflexive ACL"?

A reflexive ACL is a type of dynamic ACL that allows return traffic in response to outbound sessions initiated from within the network. This is useful for creating stateful traffic filters that only allow traffic back into the network if it was initiated from inside.

23. How would you create an ACL to block all traffic from a specific IP address?

To block all traffic from a specific IP address, you would create an ACL with a deny statement, like this:

This would deny traffic from the specified IP address.

24. What is the "time-based ACL," and when would you use it?

A time-based ACL allows you to apply ACL rules based on time conditions, such as allowing traffic only during specific hours of the day. This is useful for scenarios like restricting access to resources after business hours.

25. How do you remove an ACL from a Cisco interface?

To remove an ACL from an interface, use the following command:

This command removes the ACL from the specified direction on the interface.

26. What is a "numbered ACL" and how do you create it?

A numbered ACL is an ACL that is identified by a specific number rather than a name. To create a numbered ACL, you would use commands like:

This command creates an extended ACL with the number 101 that permits all IP traffic.

27. Can ACLs be applied to both inbound and outbound traffic on the same interface?

Yes, ACLs can be applied to both inbound and outbound traffic on the same interface, but they must be different ACLs, one for each direction.

28. How does an ACL work with NAT (Network Address Translation)?

When NAT is configured, ACLs can be used to specify which traffic should be translated. The ACL defines the traffic that is eligible for translation based on IP addresses, ports, or protocols.

29. What is the "established" keyword in an ACL?

The "established" keyword is used in an extended ACL to allow return traffic for TCP sessions that have been established. It checks for the ACK or RST bits in the TCP header to determine if the session is established.

30. How can ACLs be used to control SSH access to a router?

ACLs can be applied to the vty lines to restrict SSH access to specific IP addresses or subnets. For example:

This configuration allows SSH access only from the 192.168.1.0/24 network.

31. What is the significance of the sequence numbers in named ACLs?

Sequence numbers in named ACLs allow you to insert, delete, or reorder specific rules without having to rewrite the entire ACL. This provides greater flexibility in managing ACLs.

32. How does an "object group" work in an ACL?

An object group in an ACL allows you to group together multiple IP addresses, protocols, or ports under a single name. This simplifies ACL management by allowing you to reference the group instead of listing each individual element.

33. Can ACLs be applied to traffic between VLANs on a router?

Yes, ACLs can be applied to traffic between VLANs on a router by applying the ACL to the subinterfaces associated with the VLANs.

34. How would you apply an ACL to filter OSPF traffic?

To filter OSPF traffic, you can use an extended ACL that matches OSPF protocol traffic, and then apply it to the appropriate interface.

35. How does an ACL affect the performance of a router?

While ACLs are generally efficient, complex or poorly designed ACLs can impact router performance by increasing CPU usage, especially when dealing with a large number of rules or high traffic volumes.

Conclusion

Mastering ACLs is crucial for any network professional, especially those pursuing CCNA certification. ACLs play a pivotal role in network security and traffic management, making them a frequent topic in interviews. By understanding the intricacies of ACLs, you can demonstrate your knowledge and readiness to manage secure and efficient networks. Preparing for these questions will help you confidently tackle ACL-related queries in your CCNA interview and beyond.