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Device mtu size on a 9198 mtu switch

Posted on 2014-09-03
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Last Modified: 2014-09-08
Hello,

I have a Cisco 3850 switch with a global MTU setting of 9198.  From what I've read, you can do per port mtu settings on a 3850.  I had set the global mtu to 9198 since we are currently using the switch connected to a 1gbe SAN.

Can devices attached to the 3850 switch connect with the NIC set to MTU 1500 size?  i have VMware hosts connected to the switch with 1500 mtu set for the data nics (pnics not connected to iscsi).  They appear to be working fine for the most part but I do see slightly higher ping times to device connected to the same switch (2-8ms range).

Does the MTU size only come into effect with for devices in the same vlan?  If two devices are in the same vlan but have their nics set to 1500, they should be okay correct?
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LVL 17

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TimotiSt earned 250 total points
ID: 40303531
The default MTU on Ethernet is 1500, so everything should be working happily on that.

Jumbo frames (anything above 1500) are nice for storage traffic, as the typical 2kB or 8kB data packets will fit in one frame. As these take longer to transmit, latency might go up a bit, while throughput also increases (don't use it on the same network with VoIP :) ). These mostly work within a broadcast domain (vlan) only, as inter-vlan traffic needs routing, which might need fragmentation/reassembly, etc.

Hosts figure out each other's MTU during TCP handshake or with ICMP 'Fragmentation needed' errors, so all should just work (unless somebody firewalled every ICMP type, which is a bad idea).

Tamas
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by:rauenpc
rauenpc earned 250 total points
ID: 40304693
Well said TimotiSt. On thing I'll add in - Enable jumbo frames on a switch merely allows jumbo frames... it does not give devices any type of requirement to use jumbo frames. As long as two devices end-to-end can agree on the mtu size for communication, all is well.
For my customers, it is a standard configuration to enable jumbo frames on all switches regardless of location. This way you can always use jumbo frames if needed, but can continue to use the normal 1500 MTU for everything else.
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by:Health Payment Systems
ID: 40306244
So as long as devices in the same vlan have the same mtu, they should be okay?

What about the SVI for each vlan?  Do they need a specific "ip mtu" configured for them?
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LVL 17

Expert Comment

by:TimotiSt
ID: 40306433
Basically yes, same MTU should be fine.
IP MTU is used for routing and you most likely don't want to route jumbo frames, so leaving this on 1500 should be fine.
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