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<td bgcolor="#336666"><b><font face="Arial" size="2" color="#FFFFFF">5</font></b><font face="Arial" size="2" color="#FFFFFF"><b>.</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">5.2.3</font></b></td>
<td bgcolor="#669999"><img border="0" src="../../images/transdot.gif" width="10" height="1"></td>
<td bgcolor="#669999" height="25" width="100%"><strong>
<font face="Arial" size="2" color="#FFFFFF">Reachability issues with routing
updates in NBMA</font></strong></td>
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<font FACE="Arial" SIZE="2">By default, a Frame Relay network provides non-broadcast multi-access (NBMA)
connectivity between remote sites. An NBMA environment is viewed like other
multiaccess media environments, such as Ethernet, where all the routers are
on the same subnet. However, to reduce cost, NBMA clouds are usually built
in a hub-and-spoke topology. With a hub-and-spoke topology, the physical
topology does not provide the multi-access capabilities that Ethernet does.
<img border="0" src="../../images/1.gif" align="absmiddle" width="12" height="12"> The physical
topology consists of multiple PVCs.
<p>A Frame Relay NBMA topology may cause two problems:</p>
<ul>
<li>Reachability issues regarding routing updates </li>
<li>The need to replicate broadcasts on each PVC when a physical
interface contains more than one PVC</li>
</ul>
<p>Split-horizon updates reduce routing loops by not allowing a routing
update received on one interface to be forwarded out the same interface. If
Router B, a spoke router, sends a broadcast routing update to Router A, the
hub router, and Router A has multiple PVCs over a single physical interface,
then Router A cannot forward that routing update through the same physical
interface to other remote spoke routers. If split-horizon is disabled, then the
routing update can be forwarded out the same physical interface from which
it came. Split-horizon is not a problem when there is a single PVC on a
physical interface. This would be a point-to-point Frame Relay connection.
<img border="0" src="../../images/2.gif" align="absmiddle" width="12" height="12"></p>
<p>Routers that support multiple connections over a single physical
interface have many PVCs that terminate in a single router. This router must
replicate broadcast packets such as routing update broadcasts, on each PVC,
to the remote routers. The replicated broadcast packets can consume
bandwidth and cause significant latency to user traffic. It might seem
logical to turn off split-horizon to resolve the reachability issues caused
by split-horizon. However, not all network layer protocols allow
split-horizon to be disabled and disabling split-horizon increases the
chances of routing loops in any network.</p>
<p>One way to solve the split-horizon problem is to use a fully meshed
topology. However, this will increase the cost because more PVCs are
required. The preferred solution is to use subinterfaces. </p>
<p>
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<p><font color="#808080">
<a href="javascript:void(0)" onClick="window.open('../../elabs/mod05/elab_5_2_3/elab5_2_3.html','Interactivity','width=740,height=390,toolbar=0,resizable=yes,scrollbars=0,screenX=26,screenY=38,left=26,top=38');">
<img border="0" src="../../images/lab_icon.gif" width="25" height="25"><b></a>
<a href="javascript:void(0)" onClick="window.open('../../elabs/mod05/elab_5_2_3/elab5_2_3.html','Interactivity','width=740,height=390,toolbar=0,resizable=yes,scrollbars=0,screenX=26,screenY=38,left=26,top=38');">Lab Activity</a></b></font></p>
</font><font FACE="Arial" SIZE="2" COLOR="#808080">
<p>e-Lab Activity:
</font>
<font FACE="Arial" SIZE="2" COLOR="#808080">
Configuring Frame Relay Subinterfaces</font></p>
<font FACE="Arial" SIZE="2">
<font FACE="Arial" SIZE="2" COLOR="#808080">
<p>In this activity, the student will configure three routers in
a full-mesh Frame Relay network.</p>
<p>
<IMG alt="" height="2" width="1" src="../../images/s.gif"></p>
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<p><font color="#808080">
<img border="0" src="../../images/links_icon.gif" width="25" height="25">
<b>Web Links</b></font></p>
</font><font FACE="Arial" SIZE="2" COLOR="#808080">
<p>Problems with Running OSPF in NBMA Mode over Frame Relay<p>
<a target="_blank" href="http://www.cisco.com/en/US/tech/tk826/tk365/technologies_tech_note09186a0080094051.shtml">
http://www.cisco.com/en/US/tech/
tk826/tk365/technologies_tech_
note09186a0080094051.shtml</a></font></p>
<p>
<IMG alt="" height="2" width="1" src="../../images/s.gif"></p>
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