Understanding Cisco ACLs: Syntax And Examples - ITU Online

Understanding Cisco ACLs: Syntax and Examples

Cisco ACLs

Introduction to Cisco ACLs

In this blog, we will explore Cisco ACLs, their syntax, and provide examples to help you grasp their practical applications. Cisco, being a leading provider of networking solutions, offers various tools to manage access control effectively. In the world of networking, controlling access to resources is of utmost importance. Let learn about one such tool, Cisco Access Control Lists (ACLs).

In Cisco networking, Access Control Lists (ACLs) are used for controlling the flow of network traffic based on defined rules. ACLs act as filters that determine whether to permit or deny packets passing through a network device, such as a router or a switch. They play a crucial role in network security and traffic management by allowing or restricting access to specific resources, services, or destinations.

Here are the main purposes for which Access Control Lists are used in Cisco:

  1. Traffic Filtering: ACLs are primarily used for traffic filtering, where they examine packets and decide which ones are allowed to pass through and which ones should be blocked. By defining specific criteria in ACL rules, network administrators can control the types of traffic that can enter or exit a network.
  2. Network Security: ACLs are an essential component of network security. They can be used to protect sensitive resources and services from unauthorized access. For example, an ACL can be configured to block certain IP addresses or entire subnets known for malicious activities, effectively creating a barrier against potential threats.
  3. Access Management: ACLs enable network administrators to manage access to specific network services or applications. By permitting or denying traffic to certain ports or protocols, administrators can control who can access specific resources in the network.
  4. Quality of Service (QoS): ACLs are also used in QoS implementations to prioritize or throttle certain types of traffic. By defining ACL rules that classify packets based on specific characteristics (e.g., DSCP or IP precedence values), network administrators can ensure that critical traffic, such as VoIP or video conferencing, receives priority handling.
  5. Traffic Redirection: ACLs can be used to redirect traffic to different destinations. For example, by modifying the destination IP address of specific packets, network administrators can implement load balancing or traffic engineering solutions.
  6. Packet Counting and Monitoring: ACLs can be used to count the number of packets that match certain criteria. This feature allows network administrators to monitor the traffic flow and identify patterns or anomalies.
  7. IP Address Translation: ACLs are utilized in Network Address Translation (NAT) configurations to define which IP addresses or ranges should be translated to different addresses when traffic crosses network boundaries.

Overall, Access Control Lists are versatile tools in Cisco networking that provide granular control over traffic flow, enhance network security, and help optimize network performance. They are an integral part of building and maintaining a robust and secure network infrastructure.

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What is a Cisco ACL?

An Access Control List (ACL) in Cisco networking is a set of rules that govern the flow of data packets through a network device, such as a router or a switch. These rules define what types of traffic are allowed or denied based on specific criteria, such as source and destination IP addresses, protocols, and port numbers.

Cisco ACLs: Standard ACLs and Extended ACLs.

Cisco Standard ACL

A Cisco Standard ACL filters traffic based solely on the source IP address. It is a simple form of ACL and is often used for basic access control. The syntax for a Cisco Standard ACL is as follows:

Cisco ACL Example: Let’s say we want to deny all traffic from a specific source IP address, The ACL would be configured as follows:

Cisco Extended ACL

A Cisco Extended ACL provides more granular control as it considers multiple factors like source and destination IP addresses, protocols, and port numbers. Cisco ACL syntax for a Cisco Extended ACL is as follows:

Cisco ACL Example: Suppose we want to permit HTTP (TCP port 80) traffic from any source to a specific destination IP, The ACL would be configured as follows:

Cisco ACL List

To view the ACLs that are currently applied to an interface on a Cisco device, you can use the following command:

This Cisco ACL example will display a list of all configured ACLs along with their individual rules and hit counts (the number of times a rule has been matched).

Cisco ACL Best Practices

Cisco Access Control Lists (ACLs) are powerful tools for controlling network traffic and enforcing security policies. To ensure their effectiveness and avoid potential pitfalls, it is essential to follow best practices when configuring and managing ACLs in a Cisco networking environment. Here are several important best practices to consider:

  1. Plan and Document: Before implementing Cisco ACLs, carefully plan the desired access control policies. Understand the network’s requirements, the specific traffic that needs to be permitted or denied, and potential security risks. Document the ACL configurations, including the purpose of each rule and the rationale behind it. Proper documentation will be invaluable for troubleshooting and future updates.
  2. Specificity Over Generality: Aim for specificity in a Cisco ACL rules rather than using overly general rules. Specific rules reduce the chances of unintended consequences and make it easier to identify and resolve issues. Avoid using “any” or overly broad wildcard masks whenever possible, as they may lead to security vulnerabilities or network performance problems.
  3. Placement Matters: Apply ACLs as close to the source of the traffic as possible. Placing ACLs on inbound interfaces near the source of the traffic reduces unnecessary processing and improves overall network performance. Consider applying ACLs on the ingress interfaces of routers or switches.
  4. Regular Review and Updates: Networks are dynamic environments, and their requirements may change over time. Conduct regular reviews of ACL configurations to ensure they align with current security policies and network needs. Update Cisco ACLs promptly to reflect any changes in network topology or security requirements.
  5. Test in a Controlled Environment: Before deploying new or modified ACLs in a production environment, test them in a controlled lab or staging environment. Verify that the Cisco ACLs behave as intended and do not disrupt critical services. Testing helps avoid potential outages or security breaches resulting from misconfigurations.
  6. Logging and Monitoring: Enable logging for Cisco ACL hits to track which rules are being matched and the frequency of matches. Regularly review the logs to identify suspicious traffic patterns or to fine-tune ACL rules. Proper monitoring helps in identifying security threats and ensuring the ACLs are functioning as expected.
  7. Establish a Deny-All Rule: Always include a “deny all” rule at the end of each ACL. This rule will catch any traffic that does not match any preceding rules, ensuring that no unintended traffic is allowed through due to a lack of matching rules.
  8. Use Named ACLs: Instead of using ACL numbers, consider using named ACLs. Named ACLs provide better readability and make it easier to understand the purpose of each Cisco ACL without having to look up ACL numbers.
  9. Implement Time-Based ACLs (Optional): For added security, you can configure time-based ACLs. These ACLs define time ranges during which specific rules are active, allowing you to control traffic based on schedules.

By following these best practices, network administrators can effectively use Cisco ACLs to improve network security, optimize traffic flow, and ensure the network operates efficiently and securely. Implementing ACLs in a well-thought-out manner contributes significantly to maintaining a robust and resilient network infrastructure.


Cisco ACLs are powerful tools that play a vital role in controlling network traffic and enforcing security policies. Whether you’re restricting access to specific resources or permitting certain types of data, Cisco ACLs provide the flexibility to meet your network’s unique requirements. By understanding the syntax and best practices, you can confidently configure ACLs to enhance your network’s security and performance.

Cisco ACLs : Understanding Syntax and Examples – FAQ Section

What is a Cisco ACL and how does it function?

A Cisco Access Control List (ACL) is a set of rules used by network devices to permit or deny network traffic based on various criteria such as IP address, protocol type, or port number. Cisco ACLs function by inspecting the incoming or outgoing packets against the defined rules, and then taking action (permit or deny) based on the first match. This mechanism is crucial for network security and traffic flow management.

How do you create a basic Cisco ACL to permit or deny traffic?

To create a basic Cisco ACL, you need to enter the global configuration mode on your Cisco device and use the access-list command followed by an ACL number (for standard ACLs, use numbers 1-99 or 1300-1999; for extended ACLs, use numbers 100-199 or 2000-2699), an action (permit or deny), and the criteria (such as IP address). For example, to deny traffic from IP address, you would use: access-list 100 deny ip any. To permit all other traffic, add: access-list 100 permit ip any any.

Can Cisco ACLs be applied to both inbound and outbound traffic?

Yes, Cisco ACLs can be applied to both inbound and outbound traffic on an interface. When applied to inbound traffic, the ACL is evaluated before the traffic is routed to the outbound interface. For outbound traffic, the ACL is evaluated after the routing decision has been made. This allows for flexible control over traffic flow and security within the network.

What are some best practices for managing Cisco ACLs?

Some best practices for managing Cisco ACLs include: starting with a clear plan of what you need to achieve, using remarks to document the purpose of each entry, applying ACLs as close to the source of traffic for efficiency, using extended ACLs for granular control, regularly reviewing and updating ACLs as needed, and backing up your ACL configurations. Additionally, ordering ACL entries by the most specific to the least specific criteria can optimize performance.

How do you troubleshoot issues with Cisco ACLs?

Troubleshooting Cisco ACL issues typically involves checking the ACL syntax and ensuring it matches your intentions, verifying the application of the ACL on the correct interface and direction, using the show access-lists and show ip interface commands to review ACL entries and their hit counts, and testing traffic flow to confirm if it is being permitted or denied as expected. Debugging tools and logging can also provide insights into ACL behavior and help identify any mismatches between the ACL rules and traffic patterns.

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