Areas introduce a hierarchy to the network architecture. Another layer is added to this hierarchical structure by grouping areas into larger areas. These higher-level areas are called autonomous systems in the IP world and routing domains in the ISO world.
An autonomous system was once defined as a group of routers under a common administrative domain running a common routing protocol. Given the fluidity of modern networking life, the latter part of the definition is no longer very accurate. Departments, divisions, and even entire companies frequently merge, and networks that were designed with different routing protocols merge along with them. The result is that many networks nowadays combine multiple routing protocols with multiple degrees of inelegance, all under common administrations. So a contemporary definition of an autonomous system is a network under a common administration.
The routing protocols that run within an autonomous system are referred to as Interior Gateway Protocols (IGP). All the protocols given in this chapter as examples of distance vector or link state protocols are IGPs.
Routing protocols that route between autonomous systems or routing domains are referred to as Exterior Gateway Protocols (EGP). Whereas IGPs discover paths between networks, EGPs discover paths between autonomous systems. Examples of EGPs include the following:
- Border Gateway Protocol (BGP) for IP
- Exterior Gateway Protocol (EGP) for IP (yes, an EGP named EGP)
- The ISO’s InterDomain Routing Protocol (IDRP)
Novell also incorporates an EGP functionality, called Level 3 Routing, into NLSP.
Having given these definitions, it must be said that the common usage of the term autonomous system is not so absolute. Various standards documents, literature, and people tend to give various meanings to the term. As a result, it is important to understand the context in which one is reading or hearing the term.
This book uses autonomous system in one of two contexts:
- Autonomous system might refer to a routing domain, as defined at the beginning of this section. In this context, an autonomous system is a system of one or more IGPs that is autonomous from other systems of IGPs. An EGP is used to route between these autonomous systems.
- Autonomous system might also refer to a process domain, or a single IGP process that is autonomous from other IGP processes. For example, a system of OSPF-speaking routers might be referred to as an OSPF autonomous system. EIGRP also uses autonomous system in this context. Redistribution is used to route between these autonomous systems.
The context will indicate which form of autonomous system is under discussion at different points throughout this book.