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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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Figure 5-2 on page 173 depicts the interaction between Interior <strong>and</strong> Exterior<br />

Gateway Protocols. It shows the Interior Gateway Protocols used to maintain<br />

routing information within each AS. The figure also shows the Exterior Gateway<br />

Protocols maintaining the routing information between autonomous systems.<br />

Within an AS, multiple interior routing processes can be used. When this occurs,<br />

the AS must appear to other autonomous systems as having a single coherent<br />

interior routing plan. The AS must present a consistent view of the internal<br />

destinations.<br />

5.2 Types of <strong>IP</strong> routing <strong>and</strong> <strong>IP</strong> routing algorithms<br />

Routing algorithms build <strong>and</strong> maintain the <strong>IP</strong> routing table on a device. There are<br />

two primary methods used to build the routing table:<br />

► Static routing: Static routing uses preprogrammed definitions representing<br />

paths through the network.<br />

► Dynamic routing: Dynamic routing algorithms allow routers to automatically<br />

discover <strong>and</strong> maintain awareness of the paths through the network. This<br />

automatic discovery can use a number of currently available dynamic routing<br />

protocols. The difference between these protocols is the way they discover<br />

<strong>and</strong> calculate new routes to destination networks. They can be classified into<br />

four broad categories:<br />

– Distance vector protocols<br />

– Link state protocols<br />

– Path vector protocols<br />

– Hybrid protocols<br />

The remainder of this section describes the operation of each algorithm.<br />

There are several reasons for the multiplicity of protocols:<br />

► Routing within a network <strong>and</strong> routing between networks typically have<br />

different requirements for security, stability, <strong>and</strong> scalability. Different routing<br />

protocols have been developed to address these requirements.<br />

► New protocols have been developed to address the observed deficiencies in<br />

established protocols.<br />

► Different-sized networks can use different routing algorithms. Small to<br />

medium-sized networks often use routing protocols that reflect the simplicity<br />

of the environment.<br />

174 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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