Autonomous systems and IGP versus EGP

Interior [[routing protocols]] are designed for use in a network whose parts are under the control of a single organization. The design criteria for an interior routing protocol require it to find the best path through the network. In other words, the metric and how that metric is used is the most important element in an interior routing protocol.
An exterior routing protocol is designed for use between two different networks that are under the control of two different organizations. These are typically used between ISPs or between a company and an ISP. For example, a company would run [[BGP]], an exterior routing protocol, between one of its routers and a router inside an ISP. IP exterior gateway protocols require the following three sets of information before routing can begin:

A list of neighbor routers with which to exchange routing information.
A list of networks to advertise as directly reachable.
The autonomous system number of the local router.
An exterior routing protocol must isolate autonomous systems. Remember, autonomous systems are managed by different administrations. Networks must have a protocol to communicate between these different systems.

[[Autonomous systems]] have an identifying number, which is assigned to it by the American Registry of Internet Numbers (ARIN) or a provider. This autonomous system number is a 16-bit number. Routing protocols such as Cisco’s [[IGRP]] and [[EIGRP]] require that a unique, autonomous system number be assigned.

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