An Autonomous System (AS) is a group of networks under a single administrative control, which could be your company, a division within your company, or a group of companies. An Interior Gateway Protocol (IGP) refers to a routing protocol that handles routing within a single autonomous system. IGPs include RIP, IGRP, EIGRP, and OSPF. An Exterior Gateway Protocol (EGP) handles routing between different Autonomous Systems (AS). Border Gateway Protocol (BGP) is an EGP. BGP is used to route traffic across the Internet backbone between different autonomous systems.

Autonomous System Numbers are 16-bit numbers, allowing 65,536 possible values. The Autonomous System Number (ASN) value 0 is reserved, and the largest ASN value 65,535, is also reserved. The values, from 1 to 64,511, are available for use in Internet routing, and the values 64,512 to 65,534 is designated for private use. Routing protocols that understand the concept of an Autonomous System (AS) are IGRP, EIGRP, OSPF, IS-IS, and BGP.

Related Topics...

• Types of Routes - Static Routes and Dynamic Routes

• What is the difference between Routing Protocols and Routed Protocols

• What is Autonomous System and Autonomous System Number

• What is Administrative Distance

• Introduction to Static Routes and Default Routes

• What is Dynamic Routing and different types of Dynamic Routing

• What is Routing Metric Value