The key difference between an Access Control List (ACL) and an Access Control Entry (ACE) is that an ACL is a container or a list of rules, while an ACE is a single, specific rule within that list.
Understanding ACLs and ACEs
In network security and traffic management, ACLs are fundamental concepts used to filter or classify network traffic. They act as a policy framework that determines what traffic is permitted or denied, or how it should be handled.
Access Control List (ACL)
An ACL is essentially a collection of rules. Think of it like a comprehensive security policy applied to a specific point in the network. As the reference states, ACLs can be applied to interfaces (including LAGs), VLANs, or the Control Plane. This means an ACL can control traffic entering or leaving a physical port, traffic within a virtual network segment, or even traffic destined for the network device itself.
Access Control Entry (ACE)
An ACE is a specific instruction within an ACL. Each ACE is a granular rule that defines criteria for matching traffic and what action to take if a match occurs (e.g., permit, deny, log). The reference highlights that Access Control Entries (ACEs) are listed according to priority by sequence number and processed in lowest to highest sequence number order. This sequence is crucial because processing stops at the first matching ACE. It also notes that Each ACE attempts to match on one or more attributes of the particular traffic type, such as source IP address, destination IP address, protocol type, port numbers, etc.
How They Work Together
An ACL is empty until you add one or more ACEs to it. When network traffic passes through a point where an ACL is applied, the device evaluates the traffic against the ACEs in the ACL, starting from the lowest sequence number. The first ACE that matches the traffic determines the action taken. If no ACE matches, a default implicit rule (often a "deny all") is typically applied.
Key Differences Summarized
Here's a table outlining the main distinctions:
| Feature | Access Control List (ACL) | Access Control Entry (ACE) |
|---|---|---|
| Role | A list or container for rules | A single specific rule within an ACL |
| Composition | Composed of one or more ACEs | An individual rule item |
| Application | Applied to interfaces, VLANs, Control Plane | Part of an ACL; not applied directly |
| Processing | The overall policy applied | Processed sequentially by sequence number |
| Function | Defines the overall filtering/policy | Matches specific traffic attributes and defines an action |
Practical Insight
Imagine you want to allow HTTP traffic from a specific server while denying all other traffic from that server and denying all HTTP traffic from anywhere else.
- You would create an ACL.
- Within that ACL, you would add two ACEs:
- ACE 1 (Sequence 10): Permit TCP traffic from Server_IP to Any Destination Port 80 (HTTP).
- ACE 2 (Sequence 20): Deny IP traffic from Server_IP to Any Destination Any Port.
- You might also rely on the implicit deny-all ACE at the end of the ACL (often effectively sequence "infinity") to deny unwanted HTTP traffic from other sources.
You would then apply this ACL to the relevant interface or VLAN where the traffic enters or exits your network.
