Areas introduce a hierarchy to the internetwork 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 internetworking life, the latter part of
the definition is no longer very accurate. Departments, divisions, and even entire companies frequently
merge, and internetworks that were designed with different routing protocols merge along with them. The
result is that many internetworks nowadays combine multiple routing protocols with multiple degrees of
inelegance, all under common administrations. So a contemporary definition of an autonomous system is
an internetwork under a common administration.
NOTE
Automonous system
The routing protocols that run within an autonomous system are referred to as Interior Gateway Protocols
(IGPs). 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 (EGPs). 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 may 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 may 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 may be
referred to as an OSPF autonomous system. The chapters on IGRP and EIGRP also use
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.