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<td bgcolor="#336666"><font face="Arial" size="2" color="#FFFFFF"><b>4.</b></font><b><font face="Arial" size="2" color="#FFFFFF">3</font></b></td>
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<td bgcolor="#336666" width="100%"><b>
<font face="Arial" size="2" color="#FFFFFF">DDR Configuration</font></b></td>
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<td bgcolor="#669999" height="25"><b>
<font face="Arial" size="2" color="#FFFFFF">4.3.1</font></b></td>
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<strong><font face="Arial" size="2" color="#FFFFFF">DDR
operation</font></strong></td>
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Dial-on-demand routing (DDR) is triggered when traffic that matches a predefined set of
criteria is queued to be sent out a DDR-enabled interface. The traffic
that causes a DDR call to be placed is referred to as interesting
traffic. Once the router has transmitted the interesting traffic, the
call is terminated.<p>The key to efficient DDR operation is in the
definition of interesting traffic. Interesting traffic is defined
with the
</font><font size="2" face="Courier New">
<b>dialer-list</b></font><font size="2" face="Arial">
command. Dialer lists can allow all traffic from a specific protocol
to bring up a DDR link, or they can query an access list to see what
specific types of traffic should bring up the link. Dialer lists do not filter
traffic on an interface. Even traffic that is not interesting will be
forwarded if the connection to the destination is active.</p>
<p>DDR is implemented in Cisco routers in the following steps:</p>
<ol>
<li>The router receives traffic, performs a routing table lookup to
determine if there is a route to the destination, and identifies the
outbound interface.
<img border="0" src="../../images/1.gif" align="absmiddle" width="12" height="12"></li>
<li>If the outbound interface is configured for DDR, the router does a
lookup to determine if the traffic is interesting.
<img border="0" src="../../images/2.gif" align="absmiddle" width="12" height="12"></li>
<li>The router identifies the dialing information necessary to make
the call using a dialer map to access the next-hop router.
<img border="0" src="../../images/3.gif" align="absmiddle" width="12" height="12"></li>
<li>The router then checks to see if the dialer map is in use. If the
interface is currently connected to the desired remote destination,
the traffic is sent. If the interface is not currently connected to
the remote destination, the router sends call-setup information
through
the BRI using the D channel.
<img border="0" src="../../images/4.gif" align="absmiddle" width="12" height="12"></li>
<li>After the link is enabled, the router transmits both interesting
and uninteresting traffic. Uninteresting traffic can include data and
routing updates.</li>
<li>The idle timer starts and runs as long as no interesting traffic
is seen during the idle timeout period and disconnects the call based on the idler
timer configuration.
<img border="0" src="../../images/5.gif" align="absmiddle" width="12" height="12"></li>
</ol>
<p>The idle timer setting specifies the length of time the router should
remain connected if no interesting traffic has been sent. Once a DDR
connection is established, any traffic to that destination will be
permitted. However, only interesting traffic resets the idle timer. </p>
<p>
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<TD><IMG alt="" height="1" width="3" src="../../images/s.gif"></TD><TD align="right" valign="top">
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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>DDR<p>
<a target="_blank" href="http://www.cisco.com/en/US/tech/tk801/tk133/tech_protocol_family_home.html">
http://www.cisco.com/en/US/tech/tk801/ tk133/tech_protocol_family_ home.html</a></font></p>
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<IMG alt="" height="2" width="1" src="../../images/s.gif"></p>
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