which of the following happens by default when you create and apply a new acl on a router?

2025

2 min read 31-03-2025

which of the following happens by default when you create and apply a new acl on a router?

When you create and apply a new Access Control List (ACL) on a router, several things happen by default. Understanding these defaults is crucial for network security and proper ACL functionality. This article will clarify exactly what occurs. The key takeaway is that newly created ACLs are implicitly denied by default. Let's dive into the specifics.

Implicit Deny

The most important default behavior is the implicit deny. This means that unless explicitly permitted by a rule within the ACL, any traffic that doesn't match an existing rule is automatically denied. This acts as a crucial security safeguard, preventing unauthorized access. It's a fundamental principle of ACL design. This "deny all" behavior is applied at the end of the ACL, regardless of whether you explicitly add a "deny" statement.

ACL Ordering and Processing

Routers process ACL rules sequentially, from top to bottom. The first rule that matches a packet determines the outcome – either permitting or denying it. If a packet does not match any rule, the implicit deny at the end takes effect. Therefore, the placement of rules is critical in shaping the ACL's behavior. Consider this example:

Let's say you have an ACL with two rules:

permit ip 192.168.1.0 0.0.0.255 any
deny ip any any

A packet originating from 192.168.1.10 will be permitted because it matches the first rule. However, if the order was reversed, the packet would be denied.

Default ACL Behavior Summary

To reiterate the defaults:

Implicit Deny: The default action is to deny any traffic that does not match any rule in the ACL. This is crucial for security.
Sequential Processing: The router checks the rules sequentially; the first match decides the fate of the packet.
No Default Logging: By default, ACLs do not log denied or permitted traffic. You'll need to explicitly configure logging if you need to monitor and troubleshoot.

How to Override Defaults

While the implicit deny is often beneficial, you can customize ACL behavior. For instance, you can explicitly add permit or deny statements for specific traffic patterns. You can also enable logging to track ACL activity. These are configuration options specific to the router's operating system (e.g., Cisco IOS, Juniper JunOS).

Example Configuration (Cisco IOS)

Let's look at a basic example using Cisco IOS syntax:

access-list 100 permit ip 10.0.0.0 0.0.0.255 any
access-list 100 deny ip any any
interface GigabitEthernet0/0
ip access-group 100 in

In this configuration:

access-list 100 creates an ACL named 100.
The permit rule allows traffic from the 10.0.0.0/24 network.
The deny rule, placed after the permit rule, denies all other traffic. This is a more explicit way of demonstrating the implicit deny.
ip access-group 100 in applies the ACL to the inbound traffic of the interface.

Conclusion

Understanding the default behavior of ACLs, particularly the implicit deny and sequential processing, is critical for network administrators. By mastering these concepts, you can effectively secure your network and manage traffic flow. Remember that security is paramount, and careful planning and testing are essential when working with ACLs.