Previous chapters covered various aspects of network design, including topics such as hierarchy, summarization, specific topologies, and redundancy. This chapter changes gears and focuses on an area of increasing concern for network designers and administrators: routing protocol security. The chapter begins with a discussion of what routing protocol security is and then examines the sorts of attacks that an attacker might use against a routing system. Next, the chapter covers some protections that are available for improving the security of a routing system. Finally, the chapter concludes with some ideas about how to better protect the validity of the information within a routing system.