Mismatched MTU on Trunk Link

[mitchell helton]

Hey folks!

I'm a bit confused and need some guidance.  We have a 4510 connected to a Nexus 3064.  Our SAN, server management NICs and server iSCSI NICs will be connecting to the Nexus.  We wanted to use, or at least test, jumbo frames.  Based on the research I've done, it seems like the Nexus 3064 is all or nothing when it comes to MTU - instead of doing per interface or per VLAN.  

With that being said, my trunk port between the Nexus and 4510 is going to be mismatched... one end will have 1500 and the other will have 9000.  All the jumbo frames will be isolated to the Nexus and won't traverse the trunk link.  Is this OK?  Will it cause any problems?  Is there a better way to do this?

Thanks... please let me know if you need more info!

mitch

[Reza Sharifi]

Hi Mitch,

It should work fine.  The 5500 series are also the same.  When you enable system qos policy with jumbo frame, it applies to the entire switch.

http://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus3000/sw/qos/503_u3_1/b_Cisco_Nexus_3000_QoS_Config_Guide_Release_503_U3_1/b_Cisco_Nexus_3000_QoS_Config_Guide_Release_503_U3_1_chapter_011.html

HTH

[mitchell helton]

Thanks Reza.

Any other thoughts or opinions?  Just generally speaking, is it OK for one end of a trunk to have a different MTU than the other end?

[Joseph W. Doherty · Accepted solution]

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Posting

Mitch, NICs on the same wire can be configured to have different MTUs.  The issue to watch for is one NIC sending a frame larger than the other NIC can receive.  Unlike L3, L2's sender won't fragment, it will send the too big frames, and the receiver will drop them.

[mitchell helton]

Thanks Joe.  I was hoping you might chime in - your posts are always helpful.

Thanks to you too, Reza.