IPv4 Access Control Lists (ACLs)

Planning an ACL Application

■What are the logical points for minimizing unwanted traffic, and what ACL application(s) should be used? In many cases it makes sense to prevent unwanted traffic from reaching the core of your network by configuring ACLs to drop the unwanted traffic at or close to the edge of the network. (The earlier in the network path you can block unwanted traffic, the greater the benefit for network performance.)

■From where is the traffic coming? The source and destination of traffic you want to filter determines the ACL application to use (static port ACL, and dynamic port ACL).

■What traffic should you explicitly block? Depending on your network size and the access requirements of individual hosts, this can involve creating a large number of ACEs in a given ACL (or a large number of ACLs), which increases the complexity of your solution.

■What traffic can you implicitly block by taking advantage of the implicit deny ip any to deny traffic that you have not explicitly permitted? This can reduce the number of entries needed in an ACL.

■What traffic should you permit? In some cases you will need to explicitly identify permitted traffic. In other cases, depending on your policies, you can insert an ACE with “permit any” forwarding at the end of an ACL. This means that all IPv4 traffic not specifically matched by earlier entries in the list will be permitted.

Security

ACLs can enhance security by blocking traffic carrying an unauthorized source IPv4 address (SA). This can include:

■blocking access from specific devices or interfaces

■blocking access to or from subnets in your network

■blocking access to or from the internet

■blocking access to sensitive data storage or restricted equipment

■preventing specific IP, TCP, UDP, IGMP, and ICMP traffic types, including unauthorized access using functions such as Telnet, SSH, and web browser

You can also enhance switch management security by using ACLs to block IPv4 traffic that has the switch itself as the destination address (DA).

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