<leafNode name="fwmark"> <properties> <help>A 32-bit fwmark value set on all outgoing packets</help> <valueHelp> <format>number</format> <description>value which marks the packet for QoS/shaper</description> </valueHelp> <constraint> <validator name="numeric" argument="--range 1-255"/> </constraint> </properties> </leafNode>
So the CLI helper tells us it's a 32 bit value, but the range check only allows 8 bits.
iproute2 code revels its a 32bit unsigned integer