10.5 Using an Alternative to Access Lists
10.5.1 Configuring null interface
Access lists require CPU resources. Although the access list is optimized for router operation, there are still some cases in which the processing overhead can be diminished further. One way to eliminate access list overhead is to avoid using them. This section looks at the null interface as a suitable alternative to access lists.

Access lists are processor intensive. The router processes every line of an access list until a match is found. There is an alternative to using access lists if the policy is for unwanted traffic to a certain destination to be discarded every time. The alternative is to configure a null interface. A null interface saves CPU cycles.

The null interface is a software-only interface that functions similarly to a "null" device used by operating systems. Message traffic that is not required is directed to the null interface using a static route, where it is effectively "dropped," as shown in the main figure.

Note that it is important to consider the location of the null interface because any time a packet comes into the router to the defined destination, it will be dropped.