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    <title>Buffer-to-Buffer Credits </title>
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<a name="wp1072536"> </a><h1 class="pTitle">
Buffer-to-Buffer Credits

</h1>
<a name="wp1072537"> </a><p class="pBody">
Buffer-to-buffer credits (BB_credits) are a flow-control mechanism to ensure that Fibre Channel 
switches do not run out of buffers, so that switches do not drop frames. BB_credits are negotiated on a 
per-hop basis.

</p>
<a name="wp1072538"> </a><p class="pBody">
The receive BB_credit (fcrxbbcredit) value may be configured for each Fibre Channel interface. In most 
cases, you do not need to modify the default configuration. 

</p>
<a name="wp1072539"> </a><p class="pBody">
The receive BB_credit values depend on the module type and the port mode, as follows:

</p>
<div class="pSmartList1"><ul class="pSmartList1">
<a name="wp1072541"> </a><li class="pSmartList1">For 16-port switching modules and full rate ports, the default value is 16 for Fx mode and 255 for 
E or TE modes. The maximum value is 255 in all modes. This value can be changed as required.
</li>
<a name="wp1072543"> </a><li class="pSmartList1">For 32-port switching modules and host-optimized ports, the default value is 12 for Fx, E, and TE 
modes. These values cannot be changed.
</li>
<a name="wp1072547"> </a><li class="pSmartList1">For Generation 2, Generation 3, and Generation 4 switching modules, see the <a href="buffers05.html">&quot;Buffer Pools&quot; topic</a>.
</li>
</ul></div>
<a name="wp1072548"> </a><p class="pBodyAutonumNoRule">
<strong>Note	&nbsp;&nbsp;&nbsp;</strong> In the Cisco MDS 9100 Series switches, the groups of ports on the left outlined in white are in dedicated 
rate mode. The other ports are host-optimized. Each group of 4 host-optimized ports have the same 
features as for the 32-port switching module.

</p>
<a name="wp1099249"> </a><p class="pBodyAutonumNoRule">
<strong>Note	&nbsp;&nbsp;&nbsp;</strong> Because Generation 1 modules do not support as many buffer-to-buffer credits as Generation 4 modules 
supports, you cannot configure an ISL on E or TE ports between a Generation 1 module such as the 
16-port 1-, 2-Gbps Fibre Channel Switching Module (DS-X9016) and a Generation 4 module such as 
the 48 port 8-Gbps Advanced Fibre Channel module (DS-X9248-256K9) or the 32-port 8-Gbps 
Advanced Fibre Channel module (DS-X9232-256K9).

</p>
<hr /><br><p class="pBody">Copyright 2010-2013, Cisco Systems, Inc. All rights reserved.</p> 

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