[VLAN] Default QinQ behaviour for MTU and REORDER flag

[Peter Stuge]

On Tue, Oct 09, 2007 at 02:23:41PM +0100, Alex Zeffertt wrote:
> Some ethernet drivers implement a change_mtu() method, and use the
> MTU size passed in (from ifconfig <dev> mtu <size>) to set the
> maximum rx frame size.

MTU isn't used for rx. T for transmit.


> If a driver is not vlan aware it may set a fixed maximum rx frame
> size of 1518, or - if it implements change_mtu() - a maximum rx
> frame size of mtu + 18.
> 
> If a driver *is* vlan aware it will add 4 to the above, as it knows
> that the frames may have an extra 4 byte shim that gets stripped by
> the vlan layer before the frame enters the IP stack.
> 
> For a driver to be q-in-q aware it will need to add 4*(max q-in-q
> levels).

The driver usually doesn't have too much to say in this. The hardware
typically only supports a few maximum packet sizes and >1518 requires
varying efforts from one chip to another.


//Peter