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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>.1</b></font></td>
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<td bgcolor="#336666" width="100%"><b style="mso-bidi-font-weight:normal"><span style="font-family:Arial;mso-bidi-font-family:"Times New Roman""><o:p>
<font size="2" color="#FFFFFF">Frame Relay Concepts</font>
</o:p>
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<td bgcolor="#669999" height="25"><b>
<font face="Arial" size="2" color="#FFFFFF">5.1.5</font></b></td>
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<td bgcolor="#669999" height="25" width="100%"><strong>
<font face="Arial" size="2" color="#FFFFFF">Frame Relay address mapping and
topology</font></strong></td>
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<font FACE="Arial" SIZE="2">
When more than two sites are to be connected, consideration must be
given to the topology of the connections between them.</font><p>
<font FACE="Arial" SIZE="2">
Frame Relay is unlikely to be cost-effective when only two sites are
interconnected with a point-to-point connection. Frame Relay is more
cost-effective where multiple sites must be interconnected.</font></p>
<p>
<font FACE="Arial" SIZE="2">
WANs are often interconnected as a star topology. A central site hosts
the primary services and is connected to each of the remote sites
needing access to the services.
<img border="0" src="../../images/1.gif" align="absmiddle" width="12" height="12"> In this
hub and spoke topology the location of the hub is chosen to give the
lowest leased line cost. When implementing a star topology with Frame
Relay, each remote site has an access link to the frame relay cloud
with a single VC. The hub has an access link with multiple VCs, one
for each remote site.
<img border="0" src="../../images/2.gif" align="absmiddle" width="12" height="12"> Because
Frame Relay tariffs are not distance related, the hub does not need to
be in the geographical center of the network.</font></p>
<p>
<font FACE="Arial" SIZE="2">
A full mesh topology is chosen when services to be accessed are
geographically dispersed and highly reliable access to them is
required.
With full mesh, every site is connected to every other site. Unlike
with leased line interconnections, this can be achieved in Frame Relay
without additional hardware.
<img border="0" src="../../images/3.gif" align="absmiddle" width="12" height="12"> It is
necessary to configure additional VCs on the existing links to upgrade
from star to full mesh topology. Multiple VCs on an access
link will generally make better use of Frame Relay than single VCs.
This is
because they take advantage of the built-in statistical multiplexing. <img border="0" src="../../images/4.gif" align="absmiddle" width="12" height="12"></font></p>
<p>
<font FACE="Arial" SIZE="2">
For large networks, full mesh topology is seldom affordable. This is
because the number of links required for a full mesh topology grows at
almost the square of the number of sites. While there is no equipment
issue for Frame Relay, there is a limit of less than 1000 VCs per
link. In practice, the limit will be less than that, and larger
networks will generally be partial mesh topology. With partial mesh,
there are more interconnections than required for a star arrangement,
but not as many as for a full mesh. The actual pattern is very
dependant on the data flow requirements.</font></p>
<p>
<font FACE="Arial" SIZE="2">
In any Frame Relay topology, when a single interface is used to
interconnect multiple sites, there may be reachability issues. This is
due to the nonbroadcast multiaccess (NBMA) nature of Frame Relay.
Split horizon is a technique used by routing protocols to prevent
routing loops. Split horizon does not allow routing updates to be sent
out the same interface that was the source of the route information.
This can cause problems with routing updates in a Frame Relay
environment where multiple PVCs are on a single physical interface. </font></p>
<p>
<font FACE="Arial" SIZE="2">
Whatever the underlying topology of the physical network, a mapping is
needed in each FRAD or router between a data link layer Frame Relay
address and a network layer address, such as an IP address.
Essentially, the router needs to know what networks are reachable
beyond a particular interface. The same problem exists if an ordinary
leased line is connected to an interface. The difference is that the
remote end of a leased line is connected directly to a single router.
Frames from the DTE travel down a leased line as far as a network
switch, where they may fan out to as many as 1000 routers. The DLCI
for each VC must be associated with the network address of its remote
router. This information can be configured manually by using map
commands. It can also be configured automatically, taking LMI status
information and sending a Reverse Address Resolution Protocol (RARP)
message on each VC identified. This process is described in more
detail in a separate section.</font></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>Configuring Dynamic and Static Mapping for Multipoint Subinterfaces<p>
<a target="_blank" href="http://www.cisco.com/warp/public/125/17.html#17-A">
http://www.cisco.com/warp/public/
125/ 17.html#17-A</a></font></p>
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