Dell PowerConnect and Jumbo Frames

A friend of mine had a problem recently attaching some EqualLogic storage to some vSphere hosts using Dell PowerConnect switches. You’ll notice that it wasn’t me doing the work, so I’ve had to resort to reporting on other people doing interesting or not so interesting things. In any case, he was seeing a lot of flakiness whenever he tried to do anything with the new volumes on the ESX hosts. We went through the usual troubleshooting routine and discussed whther it was either a problem with the ESX hosts (running latest update ESX 4) or something to do with the network.

He had enabled jumbo frames all the way through (host -> switch -> array). In vSphere, you set the packet size to 9000. On the EqualLogic PS Series you set the MTU to 9000. Apparently, on the Dell PowerConnect switches, you don’t. You set it to 9216. For those of you familiar with maths, 9124 is 9 * 1024. Amazing huh? Yes, that’s right, it follows that 9000 is 9 * 1000. Okay stop now. It’s amazing that 124 could make such a difference, but, er, I guess computers need a level of accuracy to do their thing.

console# configure

console(config)# interface range ethernet all

console(config-if)# mtu 9216

console(config-if)# exit

console(config)# exit

console# copy running-config startup-config

console# exit