Jumbo Frame Question

Unanswered Question
Aug 1st, 2008

I datacenter has 2 3750s in a stack configuation. We recently purchased a SAN. I would like to use jumbo frames as the documentation for the SAN recommends. From the 3750 stack we have fiber running to our wiring closets and remote sites.

My question is, since jumbo frames are configured globally on gig interfaces, what happens when it travels over one of these fiber connections to a switch that doesn't support jumbo frames? Does the 3750 fragment the frame? Does the remote switch fragment the frame?

Thanks for your help!!


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Danilo Dy Fri, 08/01/2008 - 10:04

I'm not sure but I think fragmentation will happen in the switch that supports jumbo frame to accomodate the mtu of the switch that does not support jumbo frame.

cowetacoit Fri, 08/01/2008 - 12:05

From what i understand and the way i have our SAN configured, the SAN should be totally seperate from your LAN for security and performance reasons. Our servers have two networks configured on them one for the LAN and one for the backside SAN. Those NIC's are connected to a stack of 3750's with Jumbos, and also we have our iSCSI array attached to them. The SAN is a totally seperate network from the LAN. I would consider reworking your SAN topology.

nick.franzen Fri, 08/01/2008 - 12:11

I believe my SAN topology is acceptable according to all the documentation I have. The SAN data network is in its own VLAN separate from our current production network. The SAN Vlan is pruned to the current switch stack to prevent any excessive/unnecessary broadcast traffic.

cowetacoit Fri, 08/01/2008 - 12:14

Ok, i was wondering because you said traffic could flow over your fiber links to a different switch. I thought you were implying that those were normal LAN switches and not switches you use for your SAN.

nick.franzen Fri, 08/01/2008 - 12:16

Very true, sorry for not stating my information very clearly. What I meant is, because jumbo frames are configured globally even the non-san traffic or non-san vlans would have jumbo frames.

Sorry - I hope I didn't come off as snotty!!


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