Rosserial baudrate arduino zero

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Hi,

does someone know where the baud rate is defined? I look in ArduinoHardware.h, and in the package rosserial_python in serial_node and serial_client. I have changed them but default is always 57600. And yes I know I can define it: rosrun rosserial_python serial_node.py _port:=/dev/ttyACM0 _baud:=250000

And second: Is it correct that it works with different baudrates, not just one? And what's the maximum baudrate for an arudino zero?

Thanks