MTU of VLAN sub-interfaces

[Sandeep HM]

Hi,

I have a doubt. Why is it necessary to have "MTU of  VLAN interface must not exceed the MTU of the underlying real interface".

Is it generic to have such values or there is a specific reason?

Regards,

Sandeep

[Peter Paluch · Accepted solution]

Sandeep,

This is, I believe, because fragmentation can only occur on IP level (and even IP fragmentation is a big no-no for multilayer switches), not on Ethernet level. If a VLAN interface - as an IP interface - allowed bigger MTU than the underlying physical Ethernet interface, the resulting frame could be so large that it would exceed the MTU limitation of the physical switchport, and as a result, it would be dropped - there is no option of additionally fragmenting it.

Would this make sense?

Best regards,
Peter