DHCP options can be used to provide interface MTU to VM by vrouter.  For an existing VMI, the 'neutron port-update' command can be used to update the MTU value.

By default, there is no DHCP options configued on VMI, and the interface MTU inside a VM is 1500.

If we change the interface MTU with the command "neutron port-update --extra-dhcp-opt opt_name='interface-mtu',opt_value=' <mtu value> ' <interface> " , we can see the MTU value is changed after this interface renew its address: