Autonomous Systems

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In very large internetworks, it is necessary to divide the internetwork into separate entities known as autonomous systems, as shown in Figure 1.9. An autonomous system (AS) is a portion of the internetwork under the same administrative authority. The administrative authority can be an institution or corporation but can also be defined by the use of a routing protocol such as OSPF. The contiguous portion of an IP internetwork that is using OSPF to distribute routing information is under OSPF administrative authority and is, therefore, an OSPF AS. The AS may be further divided into regions, domains, or areas that define a hierarchy within the AS.

The protocols used to distribute routing information within an AS are known as Interior Gateway Protocols (IGPs). The protocols used to distribute routing information between ASs are known as Exterior Gateway Protocols (EGPs).

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Figure 1.9 Autonomous Systems with IGPs and EGPs

Interior Gateway Protocols

Interior Gateway Protocols (IGPs) are intra-AS routing protocols. IGPs distribute routes within the AS in either a flat or hierarchical manner.

The following are IGPs for IP internetworks:

RIP for IP . An RFC-based distance vector IGP.

OSPF . An RFC-based link state IGP.

Interior Gateway Routing Protocol (IGRP ). A distance vector IGP developed by Cisco Systems, Inc.

Exterior Gateway Protocols

Exterior Gateway Protocols (EGPs) are inter-AS routing protocols. EGPs define the way that all of the networks within the AS are advertised outside of the AS. This can include a list of network routes in a flat routing infrastructure or a list of summarized network routes in a hierarchical routing infrastructure. EGPs are independent of the IGPs used within the AS. EGPs can facilitate the exchange of routes between ASs that use different IGPs.

The following are EGPs for IP internetworks:

Exterior Gateway Protocol (EGP) . An RFC-based EGP that was developed for use between ASs on the Internet. EGP is no longer used on the Internet due to its lack of support for complex, multipath environments and Classless Inter-Domain Routing (CIDR).

Border Gateway Protocol (BGP) . An RFC-based EGP that is currently used between ASs on the Internet. BGP overcomes the weaknesses of EGP.