An autonomous system (AS) - otherwise known as a routing domain - is a collection of routers under a common administration. Typical examples are a company's internal network and an Internet service provider's network. Because the Internet is based on the autonomous system concept, two types of routing protocols are required: interior and exterior routing protocols. These protocols are:
Interior Gateway Protocols (IGP) are used for intra-autonomous system routing - routing inside an autonomous system
Exterior Gateway Protocols (EGP) are used for inter-autonomous system routing - routing between autonomous systems
The figure is a simplified view of the difference between IGPs and EGPs. The autonomous system concept will be explained in more detail later in the chapter.
Characteristics of IGP and EGP Routing Protocols
IGPs are used for routing within a routing domain, those networks within the control of a single organization. An autonomous system is commonly comprised of many individual networks belonging to companies, schools, and other institutions. An IGP is used to route within the autonomous system, and also used to route within the individual networks themselves. For example, CENIC operates an autonomous system comprised of California schools, colleges and universities. CENIC uses an IGP to route within its autonomous system in order to interconnect all of these institutions. Each of the educational institutions also uses an IGP of their own choosing to route within its own individual network. The IGP used by each entity provides best path determination within its own routing domains, just as the IGP used by CENIC provides best path routes within the autonomous system itself. IGPs for IP include RIP, IGRP, EIGRP, OSPF, and IS-IS.
Routing protocols, and more specifically the algorithm used by that routing protocol, use a metric to determine the best path to a network. The metric used by the routing protocol RIP is hop count, which is the number of routers that a packet must traverse in reaching another network. OSPF uses bandwidth to determine the shortest path.
EGPs on the other hand, are designed for use between different autonomous systems that are under the control of different administrations. BGP is the only currently-viable EGP and is the routing protocol used by the Internet. BGP is a path vector protocol that can use many different attributes to measure routes. At the ISP level, there are often more important issues than just choosing the fastest path. BGP is typically used between ISPs and sometimes between a company and an ISP


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