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Is the VLAN mtu or port mtu that my 3560 switch is actually using?

Posted on 2013-02-05
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Last Modified: 2013-02-06
I setup port-channels on the 3560X switch with multiple vlan suport and set Jumbo MTU size to 9198.  VLAN mtu is 1500.  If two devices with jumbo frame enabled (mtu=9100) are connected to this switch and are on the same VLAN, which mtu size will they be using when talking to each other?   Thank you.
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Question by:altonyoung
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by:giltjr
ID: 38857901
Simple answer, the jumbo size.

Explanation:

VLAN's don't have a MTU a VLAN interface does, ports do, and a etherchannel does.

A VLAN interface is only used when you are talking directly to the IP address for that VLAN interface.
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by:altonyoung
ID: 38858187
giltjr,

Thanks for your reply.   So if two devices on the same VLAN, but different switches with jumbo frame enabled will also talk using the jumbo size since traffic won't need to be routed out of the vlan?  

Thank you very much again for your time.
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by:Sandeep Gupta
ID: 38858735
yes..ofcourse
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giltjr earned 2000 total points
ID: 38858843
Yes, they will, assuming that the ports connecting the switches are also setup to support jumbo frames.

I also am assuming that you have setup the computers involved to use jumbo frames.  You do have to be careful if you are using Windows and the computers have two or more NIC's.

Based on my personal experience Windows will use the smallest MTU defined on any NIC no matter which NIC it is communicating over.  That is, if you have NIC1 MTU=1500 and NIC2 MTU=9000, Windows will use MTU of 1500 on both NIC's.  Don't ask me why, that is just what Windows 7 and XP did for me.
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