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+
+<head>
+<meta http-equiv="Content-Language" content="en-us">
+<meta http-equiv="Content-Type" content="text/html; charset=windows-1252">
+<title>Content</title>
+<base target="_self">
+</head>
+
+<body background="../../images/bg.gif" topmargin="0" leftmargin="0" marginheight="0" marginwidth="0" onLoad="window.focus();" link="#808080" vlink="#808080" alink="#808080">
+
+<table border="0" cellpadding="0" cellspacing="0" width="100%">
+ <tr>
+ <td bgcolor="#336666" width="18" valign="top">
+ <img border="0" src="../../images/content_lines.gif" width="16" height="25">
+ <img border="0" src="../../images/transdot.gif" width="2" height="1"></td>
+ <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>
+ <td bgcolor="#336666"><img border="0" src="../../images/transdot.gif" width="10" height="1"></td>
+ <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>
+ </span></b></td>
+ <td width="9" bgcolor="#336666"> </td>
+ </tr>
+ <tr>
+ <td bgcolor="#669999" height="25" width="18"> </td>
+ <td bgcolor="#669999" height="25"><b>
+ <font face="Arial" size="2" color="#FFFFFF">5.1.5</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">Frame Relay address mapping and
+ topology</font></strong></td>
+ <td bgcolor="#669999" height="25" width="9"> </td>
+ </tr></table>
+
+<table border="0" cellpadding="0" cellspacing="0" width="95%" bordercolor="#111111">
+ <tr>
+ <td width="15"></td>
+ <td>
+
+ <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>
+ <p>
+ <TABLE bgcolor="#B0AFAF" width="95%" border="0" cellspacing="0" cellpadding="0">
+ <TR>
+ <TD valign="top">
+ <TABLE bgcolor="#669999" width="100%" cellspacing="0" cellpadding="0" border="0">
+ <TR>
+ <TD width="5">
+ <img border="0" src="../../images/lab_toplft.gif" width="116" height="23"></TD>
+ <TD><IMG alt="" height="1" width="3" src="../../images/s.gif"></TD><TD align="right" valign="top">
+ <IMG alt="" src="../../images/corner_ur_7.gif" width="7" height="7"></TD>
+ </TR>
+ </TABLE>
+
+ <TABLE bgcolor="#B0AFAF" width="100%" cellspacing="0" cellpadding="0" border="0">
+ <TR>
+ <TD>
+ <TABLE width="100%" cellpadding="2" cellspacing="0" border="0" bordercolor="#111111">
+ <TR>
+ <TD bgcolor="#ffffff" width="15"> </TD>
+ <TD bgcolor="#ffffff"><font FACE="Arial" SIZE="2" COLOR="#000000">
+ <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>
+
+ <p>
+ <IMG alt="" height="2" width="1" src="../../images/s.gif"></p>
+ </TD>
+ </TR>
+ </TABLE>
+ </TD>
+ </TR>
+ </TABLE>
+ </TD>
+ </TR>
+ </TABLE><p>
+
+ <font face="Arial" size="2">
+ </font>
+ </td>
+ </tr>
+ </table>
+
+</body>
+
+</html>
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