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<td bgcolor="#336666"><font face="Arial" size="2" color="#FFFFFF"><b>2.</b></font><b><font face="Arial" size="2" color="#FFFFFF">3</font></b></td>
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<td bgcolor="#336666" width="100%"><strong>
<font face="Arial" size="2" color="#FFFFFF">WAN Design</font></strong></td>
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<td bgcolor="#669999" height="25"><b><font face="Arial" size="2" color="#FFFFFF">
2.3.5</font></b></td>
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<font face="Arial" size="2" color="#FFFFFF">Other layered design models</font></strong></td>
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Many networks do not require the complexity of a
full three-layer hierarchy.
<img border="0" src="../../images/1.gif" align="absmiddle" width="12" height="12"> Simpler hierarchies may be used.
<img border="0" src="../../images/2.gif" align="absmiddle" width="12" height="12"></font><p><font face="Arial" size="2">
An enterprise with several relatively small branches that require
minimal inter-branch traffic may choose a one-layer design. Historically
this has not been popular because of the length of the leased lines.
Frame Relay, where charges are not distance related, is now making this
a feasible design solution.</font></p>
<p><font face="Arial" size="2">
If there is a need for some geographical concentration, a two-level
design is appropriate. This produces a �star of stars� pattern. Again,
the pattern chosen based on leased line technology will be
considerably different from the pattern based on Frame Relay
technology.</font></p>
<p><font face="Arial" size="2">
When planning simpler networks, the three-layer model should still be
considered as it may provide for better network scalability. The hub
at the center of a two-layer model is also a core, but with no other
core routers connected to it. Likewise, in a single-layer solution the
area hub serves as the regional hub and the core hub. This allows easy
and rapid future growth as the basic design can be replicated to add
new service areas.</font></p>
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