Jumbo Frames

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May 1st, 2014
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We have a 4506 SUP7-E VSS pair running as our network core. There are several layer 3 fiber connections going out to around 25 buildings. We have a mixture of physical servers, layer 2 switches, IBM Blade Center and a Cisco UCS B series fabric interconnect pair connected to the VSS Core. We run Dell EQ iSCSI arrays that have jumbo frames enabled....which also connect to the VSS Core. We've entertained the idea of enabling jumbo frames many times but since there are so many devices that do not support jumbo frames connected we've never enabled it.

My question is what would be the implications of enabling jumbo frames on the VSS Core if there are devices that do not support it? Would the switch fragment those 9000 byte frames down to 1500 with no impact in performance? The reason we're considering jumbo frames is for obvious performance reasons on the Cisco UCS and Dell EQ iSCSI SAN. Everything we have configured is textbook except having jumbo frames enabled and we feel like that could be a missing link in the overall performance of the data center.

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Marvin Rhoads Thu, 05/01/2014 - 13:59
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There's no "autonegotiation" for jumbo frames so you need to introduce it carefully. If it's not supported along all segments of a path, the traffic will be dropped. 

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