I am planning to implement QinQ to provide Ethernet access to customer via dedicated vlan. So, I will have two .1Q tag which I will need to increase MTU from 1500 to higher on intermediate switch. I am considering to enable Jumbo frame to 9000 which is the max that all switches can support. so, I don't have change the MTU again if the frame size will increase again in the future implementation.
if you can increase the mtu on the intermediate links only I don't see any drawback in configuiring 9000 instead of 1504 (which is what you need for qinq as you need to accomodate a new 4bytes dot1q tag only.)
But if your switches support a system mtu only (hence affecting all ports) you would be changing the mtu toward the customer network too. That is not a big issue per-se but it will enable some users to send you bigger chunk of data should they find the new mtu value. Don't know if this is a issue for you but it is a possibility at least.
You can open jumbo frames to get 9000mtu and there will be no drawback about that. Because it depends on your backbone and you can limit the packet lenghts at the ingress as well. For the future applications (like more dot1q) you should prepare your backbone.
You need to restart the switches on the path for change the system mtu, so, maybe this can be count as a drawback only from the other point-of-view.
On my ME4900, "MTU" command is avaible on the each GE and TE interface and I don't need to reboot the switch to get the configured MTU on the interface while I will need to reboot the switch if i use "System MTU command".
Is "System MTU" at global and "MTU" at interface level give the same result that I can forward bigger frame size then 1500bytes?
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