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.