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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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5.2.5 Hybrid routing<br />

► Unless the network topology is fully meshed or is able to appear so, routing<br />

loops can become an issue.<br />

BGP is a popular example of a path vector routing protocol.<br />

The last category of routing protocols is hybrid protocols. These protocols<br />

attempt to combine the positive attributes of both distance vector <strong>and</strong> link state<br />

protocols. Like distance vector, hybrid protocols use metrics to assign a<br />

preference to a route. However, the metrics are more accurate than conventional<br />

distance vector protocols. Like link state algorithms, routing updates in hybrid<br />

protocols are event driven rather than periodic. Networks using hybrid protocols<br />

tend to converge more quickly than networks using distance vector protocols.<br />

Finally, these protocols potentially reduce the costs of link state updates <strong>and</strong><br />

distance vector advertisements.<br />

Although open hybrid protocols exist, this category is almost exclusively<br />

associated with the proprietary EIGRP algorithm. EIGRP was developed by<br />

Cisco Systems, Inc.<br />

5.3 Routing Information Protocol (R<strong>IP</strong>)<br />

5.3.1 R<strong>IP</strong> packet types<br />

R<strong>IP</strong> is an example of an interior gateway protocol designed for use within small<br />

autonomous systems. R<strong>IP</strong> is based on the Xerox XNS routing protocol. Early<br />

implementations of R<strong>IP</strong> were readily accepted because the code was<br />

incorporated in the Berkeley Software Distribution (BSD) UNIX-based operating<br />

system. R<strong>IP</strong> is a distance vector protocol.<br />

In mid-1988, the IETF issued RFC 1058 with updates in RFC2453, which<br />

describes the st<strong>and</strong>ard operations of a R<strong>IP</strong> system. However, the RFC was<br />

issued after many R<strong>IP</strong> implementations had been completed. For this reason,<br />

some R<strong>IP</strong> systems do not support the entire set of enhancements to the basic<br />

distance vector algorithm (for example, poison reverse <strong>and</strong> triggered updates).<br />

The R<strong>IP</strong> protocol specifies two packet types. These packets can be sent by any<br />

device running the R<strong>IP</strong> protocol:<br />

► Request packets: A request packet queries neighboring R<strong>IP</strong> devices to obtain<br />

their distance vector table. The request indicates if the neighbor should return<br />

either a specific subset or the entire contents of the table.<br />

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

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