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    <title>Buffer-to-Buffer Credit Recovery </title>
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<a name="wp1074730"> </a><h1 class="pTitle">
Buffer-to-Buffer Credit Recovery

</h1>
<a name="wp1074867"> </a><p class="pBody">
Although the Fibre Channel standards require low bit error rates, bit errors do occur. Over time, the 
corruption of receiver-ready messages, known as R_RDY primitives, can lead to a loss of credits, which 
can eventually cause a link to stop transmitting in one direction. The Fibre Channel standards provide a 
feature for two attached ports to detect and correct this situation. This feature is called buffer-to-buffer 
credit recovery.

</p>
<a name="wp1074868"> </a><p class="pBody">
Buffer-to-buffer credit recovery functions as follows: the sender and the receiver agree to send 
checkpoint primitives to each other, starting from the time that the link comes up. The sender sends a 
checkpoint every time it has sent the specified number of frames, and the receiver sends a checkpoint 
every time it has sent the specified number of R_RDY primitives. If the receiver detects lost credits, it 
can retransmit them and restore the credit count on the sender.

</p>
<a name="wp1074869"> </a><p class="pBody">
The buffer-to-buffer credit recovery feature can be used on any non arbitrated loop link. This feature is 
most useful on unreliable links, such as MANs or WANs, but can also help on shorter, high-loss links, 
such as a link with a faulty fiber connection.

</p>
<a name="wp1074870"> </a><p class="pBodyAutonumNoRule">
<strong>Note	&nbsp;&nbsp;&nbsp;</strong> The buffer-to-buffer credit recovery feature is not compatible with the distance extension (DE) feature, 
also known as buffer-to-buffer credit spoofing. If you use intermediate optical equipment, such as 
DWDM transceivers or Fibre Channel bridges, on ISLs between switches that use DE, then 
buffer-to-buffer credit recovery on both sides of the ISL needs to be disabled.

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

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