Routing TCP/IP, Volume I (CCIE Professional Development)

Appendix B. Tutorial: Access Lists

Access lists are probably misnamed these days. As the name implies, the original intention of an access list was to permit or deny access of packets into, out of, or through a router. Access lists have become powerful tools for controlling the behavior of packets and frames. Their use falls into three categories (Figure B.1):

• Security filters protect the integrity of the router and the networks to which it is passing traffic. Typically, a security filter permits the passage of a few, well-understood packets and denies the passage of everything else.

• Traffic filters prevent unnecessary packets from passing onto limited-bandwidth links. These filters look and behave much like security filters, but the logic is ...