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<td bgcolor="#336666"><font face="Arial" size="2" color="#FFFFFF"><b>5.</b></font><b><font face="Arial" size="2" color="#FFFFFF">2</font></b></td>
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<td bgcolor="#336666" width="100%"><b>
<font face="Arial" size="2" color="#FFFFFF">Configuring Frame Relay</font></b></td>
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<td bgcolor="#669999" height="25"><b>
<font face="Arial" size="2" color="#FFFFFF">5.2.4</font></b></td>
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<td bgcolor="#669999" height="25" width="100%"><strong>
<font face="Arial" size="2" color="#FFFFFF">Frame Relay subinterfaces</font></strong></td>
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<p><font size="2" face="Arial">
To enable the forwarding of broadcast routing updates in a hub-and-spoke
Frame Relay topology, configure the hub router with logically assigned
interfaces. These interfaces are called subinterfaces. Subinterfaces are logical subdivisions of
a physical interface.
<img border="0" src="../../images/1.gif" align="absmiddle" width="12" height="12"></font><p><font size="2" face="Arial">
In split-horizon routing environments, routing updates received on one
subinterface can be sent out another subinterface. In a subinterface
configuration, each virtual circuit can be configured as a point-to-point
connection. This allows each subinterface to act similarly to a leased
line. Using a Frame Relay point-to-point subinterface, each pair of the
point-to-point routers is on its own subnet.</font><p><font size="2" face="Arial">
Frame Relay subinterfaces can be configured in either point-to-point or
multipoint mode:</font><ul>
<li><font size="2" face="Arial">
<b>Point-to-point</b> � A single point-to-point subinterface is used to establish
one PVC connection to another physical interface or subinterface on a remote
router. In this case, each pair of the point-to-point routers is on its own
subnet and each point-to-point subinterface would have a single DLCI. In a
point-to-point environment, each subinterface is acting like a
point-to-point interface. Therefore, routing update traffic is not subject
to the split-horizon rule.</font></li>
<li><font size="2" face="Arial">
<b>Multipoint</b> � A single multipoint subinterface is used to establish multiple
PVC connections to multiple physical interfaces or subinterfaces on remote
routers. All the participating interfaces would be in the same subnet. The
subinterface acts like an NBMA Frame Relay interface so routing update
traffic is subject to the split-horizon rule.</font></li>
</ul>
<p><font size="2" face="Arial">
The </font><b><font size="2" face="Courier New">
encapsulation frame-relay</font></b><font size="2" face="Arial">
command is assigned to the physical interface.
All other configuration items, such as the network layer address and DLCIs,
are assigned to the subinterface.</font></p>
<p><font face="Arial" size="2">
Multipoint configurations can be used to conserve addresses that can be
especially helpful if Variable Length Subnet Masking (VLSM) is not being
used. However, multipoint configurations may not work properly given the
broadcast traffic and split-horizon considerations. The
point-to-point subinterface option was created to avoid these issues.</font></p>
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<b>Web Links</b></font></p>
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<p>Configuring Frame Relay Subinterfaces<p>
<a target="_blank" href="http://www.cisco.com/en/US/tech/tk713/tk237/technologies_configuration_example09186a008009421e.shtml">
http://www.cisco.com/en/US/tech/</a><br>
<a target="_blank" href="http://www.cisco.com/en/US/tech/tk713/tk237/technologies_configuration_example09186a008009421e.shtml">
tk713/tk237/technologies_configuration_</a><br>
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example09186a008009421e.shtml</a></font></p>
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