ACL

ACL

Access Control Lists (ACLs) are an important aspect of network security that allow administrators to control access to network resources. As a network engineer, it is important to have a clear understanding of ACLs, including the differences between standard and extended ACLs, and their use cases. In this blog post, we will explore these key points in detail.

What is ACL?

An ACL is a list of rules that determines whether or not to permit or deny access to a particular network resource. These rules are processed in a sequential manner, and the first rule that matches the source address and port of a packet is applied. Depending on the result of this rule, the packet is either forwarded or dropped.

Differences between standard vs extended ACLs:

Standard ACLs:

• Only match the source IP address of a packet
• Can only be used to filter traffic based on source IP addresses
• They are easier to configure and understand compared to extended ACLs

Extended ACLs:

• Match source and destination IP addresses, as well as protocol type and port numbers
• Offer more granular control over network traffic compared to standard ACLs
• They are more complex to configure compared to standard ACLs

Use cases of different types of ACLs:

Standard ACLs are commonly used to control access to specific network resources, such as blocking access to a server from specific IP addresses.

Extended ACLs are used for more advanced filtering, such as allowing access to specific services (e.g. HTTP or FTP) from specific IP addresses. They are also used to control access to specific applications, such as allowing only specific users to access a company's VPN.

Configuration steps for each type of ACL:

Standard ACL configuration steps:

1. Enter into global configuration mode
2. Create an ACL using the "access-list [number] [permit/deny] [source IP]" command
3. Apply the ACL to an interface using the "ip access-group [number] [in/out]" command

Extended ACL configuration steps:

1. Enter into global configuration mode
2. Create an ACL using the "access-list [number] [permit/deny] [protocol] [source IP] [destination IP] [source port] [destination port]" command
3. Apply the ACL to an interface using the "ip access-group [number] [in/out]" command

Show commands to validate:

1. Show the ACL configuration using the "show access-lists" command
2. Verify the ACL is applied to an interface using the "show run" command
3. Monitor the effectiveness of the ACL using the "show access-lists [number]" command

In conclusion, ACLs play a crucial role in network security by controlling access to network resources. Understanding the differences between standard and extended ACLs, and their use cases, is essential for CCNP certified engineers. The above configuration steps and show commands provide a comprehensive guide to implementing and validating ACLs in a network.

Top interview questions on ACL

1. What is an Access Control List (ACL)? Answer: An Access Control List (ACL) is a list of rules that control access to network resources by determining whether to allow or deny access based on the source address and port of a packet.

2. What is the difference between a standard and an extended ACL? Answer: A standard ACL only matches the source IP address of a packet and can only be used to filter traffic based on source IP addresses. An extended ACL matches both source and destination IP addresses, as well as protocol type and port numbers, and offers more granular control over network traffic.

3. What is the purpose of applying an ACL to an interface? Answer: The purpose of applying an ACL to an interface is to control the flow of incoming or outgoing traffic to and from the network.

4. How is the order of rules in an ACL determined? Answer: The order of rules in an ACL is determined by the order in which they are configured. Rules are processed in a sequential manner, and the first rule that matches the source address and port of a packet is applied.

5. What happens if there is no match in an ACL? Answer: If there is no match in an ACL, the default action is to deny access to the network resource.

6. Can you explain the command to create a standard ACL? Answer: The command to create a standard ACL is "access-list [number] [permit/deny] [source IP]".

7. Can you explain the command to create an extended ACL? Answer: The command to create an extended ACL is "access-list [number] [permit/deny] [protocol] [source IP] [destination IP] [source port] [destination port]".

8. How do you apply an ACL to an interface? Answer: An ACL is applied to an interface using the "ip access-group [number] [in/out]" command.

9. What is the purpose of the "show access-lists" command? Answer: The "show access-lists" command is used to display the configuration of all ACLs in a network.

10. How can you monitor the effectiveness of an ACL? Answer: The effectiveness of an ACL can be monitored using the "show access-lists [number]" command, which displays the hit count for each rule in the ACL.