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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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infinity. To minimize this exposure, whenever a network is unavailable, the<br />

incrementing of metrics through routing updates must be halted as soon as it is<br />

practical to do so. In a R<strong>IP</strong> environment, costs continue to increment until they<br />

reach a maximum value of 16. This limit is defined in RFC 1058.<br />

A side effect of the metric limit is that it also limits the number of hops a packet<br />

can traverse from source network to destination network. In a R<strong>IP</strong> environment,<br />

any path exceeding 15 hops is considered invalid. The routing algorithm will<br />

discard these paths.<br />

There are two enhancements to the basic distance vector algorithm that can<br />

minimize the counting to infinity problem:<br />

► Split horizon with poison reverse<br />

► Triggered updates<br />

These enhancements do not impact the maximum metric limit.<br />

Split horizon<br />

The excessive convergence time caused by counting to infinity can be reduced<br />

with the use of split horizon. This rule dictates that routing information is<br />

prevented from exiting the router on an interface through which the information<br />

was received.<br />

The basic split horizon rule is not supported in RFC 1058. Instead, the st<strong>and</strong>ard<br />

specifies the enhanced split horizon with poison reverse algorithm. The basic<br />

rule is presented here for background <strong>and</strong> completeness. The enhanced<br />

algorithm is reviewed in the next section.<br />

The incorporation of split horizon modifies the sequence of routing updates<br />

shown in Figure 5-7 on page 186. The new sequence is shown in Figure 5-8. The<br />

tables show that convergence occurs considerably faster using the split horizon<br />

rule.<br />

Time<br />

D: Direct 1 Direct 1 Direct 1 Direct 1<br />

B: Unreachable Unreachable Unreachable C 12<br />

C: B 3 A 4 D 11 D 11<br />

A: B 3 C 4 Unreachable C 12<br />

Note: Faster Routing Table Convergence<br />

Figure 5-8 Network convergence with split horizon<br />

Chapter 5. Routing protocols 187

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