Mixing jumbo frames with regular traffic

[CrackedJack1]

In a nutshell, does allowing jumbo frames on a port make everything use jumbo on it or does it only allows for the possibility for devices to send larger frames without getting split?

Details:

I have some ESXi servers and storage connected to a 4500x. The 4500x is linked to other switches. The switch has mtu set to 9216 on the ports

On the ESXi servers, I have a distributed switch which has vlans for regular traffic and iscsi traffic. The mtu on the switch and port group for iscsi is set to 9000. All the vms have their mtu at the default for the OS (so 1500).

I have been careful to not allow 9000 traffic to cross a path with an mtu of 1500.

So my question is does having traffic with an mtu of 1500 going through ports with an mtu of 9000 cause any problem? Does my 1500 get changed to 9000 somehow or am I ok ? Does setting a higher mtu only allow for larger packets to pass without getting fragmented but doesn't affect the original packet?

Thanks 

[Joseph W. Doherty · Accepted solution]

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Posting

Enabling MTUs greater that 1500, allows larger L2 frames.  If such hits an egress L3 IP interface, that's smaller, it will be fragmented.  If such hits an ingress or egress L2 interface, that's smaller, it will be dropped.

1500 MTU shouldn't have any issues on interfaces configured for larger MTU.  No, such frames are not changed on interfaces with larger MTUs (just as a frame of 64 bytes isn't changed when it crosses an interface with a MTU of 1500).

[CrackedJack1]

Thanks for the explanation, especially the L2/L3 stuff. I've seen the splitting/dropping behavior  before but didn't make the L2/L3 connection in my head.