Cisco CCNA Exam Tutorial Configuring Standard Access Lists
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Cisco CCNA Exam Tutorial: Configuring Standard Access Lists
Overview:
Mastering Access Control Lists (ACLs) is essential for passing your Cisco exams. Learn the basics from Chris Bryant, CCIE12933, and elevate your skills.
Understanding ACLs:
Access Control Lists (ACLs) are crucial tools for network management, allowing routers to permit or deny packets based on specific criteria. Although configured globally, ACLs are enforced at the interface level using the `ip access-group` command. You can filter packets as they enter or exit an interface.How ACLs Work:
When a packet encounters an interface with an applied ACL, it is checked against each line of the ACL until a match is found. The router takes action based on this match. If no match occurs, all packets are subject to an implicit deny, blocking them. This default behavior of ACLs is fixed and cannot be changed.Types of ACLs:
- Standard ACLs: Evaluate only the source IP address.- Extended ACLs: Examine both source and destination IPs and can consider port numbers.
The numerical ranges are as follows:
- Standard ACLs: 1-99 and 1300-1399
- Extended ACLs: 100-199 and 2000-2699
Important Points:
- Source IP Focus: Standard ACLs only consider the source IP.- Order Matters: ACL lines are evaluated from top to bottom, so ensure the order is correct.
- Implicit Deny: Any packet not explicitly permitted by an ACL is implicitly denied.
Configuring a Standard ACL:
To allow only packets from the source network 172.12.12.0 on Router 3's Ethernet interface, configure the ACL as follows:```plaintext
R3
conf t
R3(config)access-list 5 permit 172.12.12.0 0.0.0.255
```This ACL includes one line that explicitly allows packets from the specified source. An implicit deny blocks all others not matching this criterion.
Applying the ACL:
After configuration, apply the ACL to the Ethernet0 interface:```plaintext
R3
conf t
R3(config)interface e0
R3(config-if)ip access-group 5 in
```Checking Existing ACLs:
Before creating new ACLs, it's wise to review existing ones on the router:```plaintext
R1
show access-list
```Example output:
- Standard IP access list 1: `permit 0.0.0.0`
- Standard IP access list 5: `permit 172.1.1.1`
- Extended lists include specific protocol rules.
Conclusion:
As you progress through Cisco certifications and your networking career, ACLs will become a vital tool. Understanding how to write and apply them begins with mastering these foundational concepts.Embark on your journey to mastery today!
You can find the original non-AI version of this article here: Cisco CCNA Exam Tutorial Configuring Standard Access Lists.
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