Chapter 4. Interdomain Routing Basics

This chapter covers the following key topics:

• Overview of routers and routing— Provides a brief consideration of basic routing and interior gateway protocols (IGPs) as a point of contrast for the next chapter's more in-depth deliberation of exterior gateway protocols.

• Routing protocol concepts— This section provides an overview of the distance vector and link-state distributed routing algorithms.

• Segregating the world into autonomous systems— An autonomous system is a set of routers that shares the same routing policies. Various configurations for autonomous systems are possible, depending on how many exit points to outside networks are desired and whether the system should permit transit traffic.

The Internet is a conglomeration of autonomous systems that define the administrative authority and the routing policies of different organizations. Autonomous systems are made up of routers that run Interior Gateway Protocols (IGPs) such as Routing Information Protocol (RIP), Enhanced Interior Gateway Routing Protocol (EIGRP), Open Shortest Path First (OSPF), and Intermediate System-to-Intermediate System (IS-IS) within their boundaries and interconnect via an Exterior Gateway Protocol (EGP). The current Internet de facto standard EGP is the Border Gateway Protocol Version 4 (BGP-4), defined in RFC 1771^[1].