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I am hoping one of you can help clear up some of my confusion on this matter.

We have 2 new catalyst 3750 switches to support our VM hosts and SAN arrays. The SAN arrays utilize jumbo frames with an MTU of 9000.

In the switch config- we specified a global jumbo MTU of 9000. This is a Gigabit switch so to my understanding the Gigabit port will accept the frames on ingress- but will drop the frame on egress if the port is not configured to forward jumbo frames. We did not configure the MTU on a port basis but globally with "system mtu jumbo" command. I'm assuming these gigabit ports utilize this command and be able to send/receive jumbo frames?

Additionally- We have VMhosts which will reside on a different network as they are physically in a nother location. The frames will be fragmented if needed at layer 3- is it worth configuring the hosts for jumbo frames considering the fragmentation?

Is it worth it to have jumbo frames configured if a host is set with an MTU of 1500 and the SAN 9000?

Mike P
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Jumbo frames need to be enabled end-to-end to have value. I don't think they'll buy you anything over the L3 link, and the hosts that are connecting to the SAN definitely need to have jumbo frames enabled or they won't be used.

That said, I haven't found jumbo frames to have an appreciable performance impact, but you'll have to test in your environment to see if it helps.

scottm32768
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  • Thanks for the info. now one more question ontop of that- if the switch interface is not configured for jumbo frames but has the global jumbo MTU set- is there any chance of us losing connectivity due to improper MTU settings or will it always send at 1500 if not configured? – Mike P Mar 18 '13 at 21:08
  • You cannot set the mtu on individual interfaces. The global command is the only option for this platform. Here's details: http://www.cisco.com/en/US/products/hw/switches/ps700/products_configuration_example09186a008010edab.shtml#c3 – scottm32768 Mar 19 '13 at 01:38